Your options are few.
Your low mental stats means you can't multiclass to anything particularly exciting. Ranger, for example, requires 13 Wis. Unless your GM is waiving these requirements, options like going Sorcerer for Quickened Booming Blades (to punish foes if they try to move away and get an extra attack) or Druid (to be a raging bear tank) are not available to you.
For Your ASI: Sentinel. Your class ability to reduce incoming damage by 2d6 is not hugely meaningful at this level and it will get less and less meaningful as time goes on and enemy damage goes up. Using a Reaction to stop someone from moving for that turn is far more meaningful, as a big bad melee monster cannot now go and attack your Bard 3 times for 1d10+9 with additional poison, he must instead attack you.
Likewise - I am unimpressed with the class abilities barbarian has given you at 6th and 7th level. Ask your GM if you can retrain those to another class.
Classwise, your GM waiving multiclassing requirements would deeply impact you. Your options would be many, and seriously change the way you play your barbarian. As is, without investing 2 ASI's into a mental stat, the only two classes you can multiclass to are fighter or rogue. Let me repeat that - the ONLY two classes you can multiclass to are fighter or rogue.
Of those two, you pick Fighter.
People will often suggest fighter because of Action Surge. Fighter is in general just generically good and people therefore suggest it as default. However here, i'd suggest other things, but you aren't allowed to take those other things, so.
Why Fighter? Two things.
Tunnel-Fighting (UA, Fighting Style) - Your question mentions the Revised Ranger, which is Unearthed Arcana and not from a published book. Therefore I will assume that provisionally, UA content may be allowed at your table. Tunnel Fighting is perfect for tanks with the Sentinel feat. It allows you to make opportunity attacks without using up your Reaction, by expending your bonus action to enter a specific stance (if you move you must re-enter the stance). Likewise it lets you use up your reaction to hit someone who moved even inside your reach. This makes your tanking a lot stronger and better. Potentially infinite enemies could get hit this way if they all have to move 'past' you and you hit all your attacks (the classic 'man in a tunnel' scenario this fighting style refers to).
Echo Knight (Sub-Class) - This sub-class allows you to expend your bonus action to create an 'Echo' in a location. This Echo can take opportunity attacks for you if enemies leave its threatened area (as if it were you). This synergizes with Tunnel-Fighter & Sentinel heavily, by creating two 'threatened zones' where enemies basically cannot leave without first destroying the Echo (it has 1 hp, but you can create it again on your turn) or knocking you unconscious.
Echo Knight also lets you 'Unleash Incarnation' to get an extra attack on an Attack action, which is okay, Con times per long rest, which is better as it uses your humongous Con stat for something.
You can't set up your Tunnel Fighting stance and create the Echo on the same turn, as both use your bonus action. But creating the Echo may let you avoid moving - as you can attack and take Opportunity Attacks from it, thus letting you preserve your Tunnel Fighting stance for longer - or the fragility of the Echo may mean you prefer to move up and activate tunnel stance personally. Either way, if you can stand still, you can extend your 'zone of no u' by creating the Echo on your 2nd turn, doubling your potential for stopping people getting past you to the squishy casters.
If Tunnel Fighter is not allowed, you instead take Dueling. +2 to Damage isn't nothing (if you use a shield). Otherwise you take Superior Technique, and learn Goading Attack and the one that lowers enemy move speed.
If Tunnel Fighting isn't allowed, you may wish to take Battle Master instead of Echo Knight. It depends entirely on how many short rests your party typically gets/takes between encounters. Battle Master's superiority dice can be used to taunt enemies or lower their move speed, or give disadvantage (such as on your second attack/second target - the first already gets disadvantage from ancestral guardians), as well as adding to damage, and they recharge on short rests. Echo knight extends the range of your Sentinel reaction attacks to give you more options there, and gives you extra attacks per long rest. If using a 2-handed weapon, echo knight will be a bit better, if sword and board, battle master a bit better, etc, but it's all very minor at this stage.
Your real best option is tunnel fighter and the ability to become a wall that enemies must overcome before they can get close to your allies.
After level 3, Fighter doesn't grant many class features you care about. At level 4, you get another ASI, and can spend it on something. Strength, perhaps. None of the feats are super useful to you after Sentinel. Great Weapon Mastery is the only exception - with Reckless Attack, you've got a chance of hitting vs the lower-AC enemies even with the -5, but it means you can't use a shield.
You could multiclass to Rogue if you wanted to get expertise in some skills (like Athletics, perhaps - if you planned to use the Grappling battle master maneuver to grab onto people after hitting them, or something) after level 3, or you could take more barbarian levels, it doesn't really matter. Out of the classes available to you, the meaningful abilities are largely exhausted unless you plan to go many more levels past 12.