I've only looked at single-class options, but this is an overview of the main obvious classes. All the classes I've skipped are because they're very short on healing and buff options; I made an exception for Barbarian because I think its tank/leader options might not be obvious!
I've gone into some detail on the points you wanted for the classes that are most designed for this role, and touched on others that can be adapted into it. This is a basic overview - there are almost certainly some tricks and optimisations that will make something I haven't considered a much better option than I realise.
I'm focusing on the Player's Handbook and Xanathar's Guide to Everything options, plus of course the Artificer from Eberron: Rising from the Last War.
Artificer: the Battlesmith
Keeping allies alive and effective: The Artificer spell list gives you access to Cure Wounds and condition-removal spells like Lesser Restoration. You’re a half-caster much like the Paladin, so you have a decent amount of spell slots, but not as many as a specialist like a Cleric. The Battlesmith subclass adds Aura of Vitality and Aura of Purity to your spell list, giving you some very strong options from 9th level onwards.Your Steel Defender can give enemies disadvantage on attacks, like the Protection fighting style, and has its own Reaction to do so; it also gives you additional healing from 9th level onward.
Improving their abilities: This is a major theme for the Artificer! Your spell list has options like Aid, Enlarge/Reduce and Magic Weapon; you can give allies effectively permanent buffs with your Infusions; and the Spell-Storing Item feature at 11th level lets you hand out spellcasting options to party members who don’t have as much to do in combat.
Toughness: Medium armour and shields give you decent AC, but it’s not spectacular, and with d8 hit points per level you’re not winning any prizes. The Steel Defender helps a lot, though - at 5 HP per Artificer level, it’s not going to be far behind your own HP pool, and its Repair feature and the fact that you can heal it for no resources with Mending means it should always be at full if you have time to look after it. Also, you have Shield as a Battlesmith spell!
Threat: You and the Steel Defender both have decent attacks, and with a few Smite spells on your list, you can do something there. You’ve got cantrips and Extra Attack, so you can be flexible here. The fact that you get to use Int for attack and damage rolls is a major bonus, as it means not having to choose between your attacks and your spells when assigning ability scores. That will mean some difficult choices if you’re playing from 1st level, as you only get the feature at 3rd.
Flavour: It’s Eberron and you’re considering a Warforged. I can’t not mention it.
The Artillerist subclass is much more focused on damage than support, but the Alchemist is also worth a look; it trades off a lot of the tanking for much improved healing and support options. I don’t think it has the balance you’re looking for, but I’d suggest having a look to draw your own conclusion. If Unearthed Arcana is an option, the recent Armorer subclass is also worth looking into - it obviously improves your defences, but it also gives some tank-style control tools.
Keeping allies alive and effective: You have a healing-focused spell list, and a lot of very flexible healing in Lay on Hands. Later on, your various aura abilities will also help - adding Charisma to saving throws can go a long way.
Improving their abilities: Spells like Bless, Heroism, Aid and Magic Weapon carry a lot of weight here, and Crusader’s Mantle appears later on if your party make a lot of attacks.
Toughness: d10 hit points, heavy armour, and options like Shield of Faith mean you’re very resilient.
Threat: Smite gives you a strong and very reliable way to apply damage, on top of personal buffs like Divine Favour.
Oath of Devotion: (PHB) A Channel Divinity that makes your attacks scarily accurate for a minute, multiplying the benefit of spells like Divine Favour, and its spell list is a good mix of support spells.
Oath of Ancients: (PHB) Some control options and makes nearby allies very resilient against spells.
Oath of Redemption: Absolutely fantastic for channelling damage onto you once you hit level 7, and if you go to level 15, you get free healing every turn for the low price of having managed to lose half your hit points. Its spell list is less support-oriented than you might expect, being focused more on ending conflict without violence, but with the base Paladin list that isn't a huge problem.
Vengeance (PHB) is more focused on offence, and Conquest (XGtE) focuses on control, which are both less your brief (unless you think of control as a form of tanking).
Keeping allies alive and effective: A full caster with a fair amount of healing on the list, so while your class features don’t add much during combat, they don’t need to. Song of Rest makes short rests better, too.
Improving their abilities: You have Bardic Inspiration, but not as much else as you might expect - the spell list does more illusion and control than enhancing your allies.
Toughness: d8 hit dice and at best medium armour - not terrible, but the worst of anything I’ll be suggesting here.
Threat: Not a huge amount in the base class, though I’ll note you can use Magical Secrets to steal some impressive damage spells at level 10.
Subclass options: Valour (PHB) makes you a straightforward fighter-ish type and adds damage and AC-buffing to Inspiration; Swords (XGtE) gives more damage, mobility and some tricks. The others are much more caster-focused than the frontline style you want, and even these two are likely to be a touch less resilient than you want.
Keeping allies alive and effective: The spell list excels, and you’re a full-progression caster. No relevant class features outside the subclasses.
Improving their abilities: You have basically all the buff spells!
Toughness: Not the worst - d8 hit dice, shield, medium armour (probably heavy from subclass), but on the lower end of the recommendations I’ll be making.
Threat: There’s some good options in the spells here. Spirit Guardians is worth singling out - for a front-line character, damaging every enemy who approaches or starts their turn within 15 feet is incredible.
Subclass options: Let’s go down in detail. I’ll skip most of the ones that don’t feel relevant. Note that the caster types can do fine in melee using cantrips like Sacred Flame or XGtE’s Word of Radiance for damage.
Life: Excels at healing, in return for not doing a lot else. You might want something a bit tankier, but it could do what you want!
Light: Sure it’s a caster type, but it gives you a defensive reaction that, from 6th Level, you can use for other people too; and Radiance of the Dawn is good if you dive into the middle of all your enemies. Probably not but maybe!
Nature: Adds Barkskin, which rounds out the buff options a little if you have an ally with a conspicuously low AC, and Dampen Elements at level 6 is a good defensive option against elemental damage.
Tempest: Makes you much, much scarier, and adds some pushing options to your powers.
War: All its spells are exactly the sort of thing you want, it makes you more potent at attacking, and has a couple of class feature options for buffing allies too. Then at level 17 you become resistant to some attacks, though only nonmagical ones.
Forge: Very tanky - you get AC bonuses and fire resistance at level 6, and at 17, immunity to fire on top of what War gets. The spells are a mix of offence, buffs and utility.
Grave: This one might be interesting. You’re much better at healing allies who have dropped, which is a bit “I’ve failed as a tank already,” but might be fun to work with. Spells are mostly debuffs, but the rest of the class features actually hit the kind of leader/tank mix you want. It doesn’t give heavy armour and improves cantrips rather than attacks, so it’s more designed for casting than frontline.
Some other options that are worth looking at but don’t really hit everything you wanted:
Very tough (don’t forget that Rage gives resistance to bludgeoning, piercing and slashing, which is very strong and notably doesn't have the "nonmagical" clause several similar features I've mentioned have), excellent damage. No healing and doesn’t hit your Leader goals, but some of the subclasses are relevant by adding a little tanking:
Totem Warrior (PHB): The level 3 and level 14 options both have a tank option under Bear (resistance to all but psychic followed by disrupting enemies attacking your allies) and a leader-type option under Wolf (giving allies advantage on attacks against enemies you engage and knocking enemies prone, which disrupts their attacks and makes them easier to stab). You can mix and match these, too - you can pick the Bear ability at 3rd and the Wolf one at 14th or vice-versa, so there's some space to pick your favourite options here.
Ancestral Guardian (XGtE): Very good tank options - it lets you hinder enemies’ attacks and, from level 14, punish them for attacking your allies.
Storm Herald (XGtE): Particularly Tundra, which gives allies temporary hit points and lets you immobilise enemies at level 14.
Zealot might be worth a look, but I don’t think its one Leader feature is quite what you want.
Druid, Circle of the Moon
This one’s worth mentioning but I don’t have enough knowledge of the options to give a good answer - I’d have to look at a lot of beast statblocks to work out if any do what you want. Spells have some decent buff options and healing, and the “free” HP from Wild Shape makes you very resilient.
Warlock, Celestial Patron
Full caster (ish) healing that replenishes every short rest, and some additional healing from a class feature. Not a lot of buffs, even with your additional class spells. Reasonable HP and some relevant defensive things, though - Mirror Image, Armour of Agathys and Hellish Rebuke seem notable - and Pact of the Blade gives you front-line options. Invocations like Armor of Shadows for permanent Mage Armor and Thirsting Blade and Eldritch Smite for damage are relevant. I'll also mention multiclassing - if you take another class that gives access to Cure Wounds, you can do almost as much healing as a Celestial Patron warlock just from the spell slots, and that frees you up to consider Hexblade's much better frontline potential, or the Fiend for temporary hit points and flexible resistance.