Can I combine Haste, Green Flame Blade, Divine Smite, and Thunderous Smite like this?

Question

This question is asking if my coup de grace manoeuvre action-sequence, that could only be pulled off once per long rest, is working as intended. I'll try to keep what I am trying to do as simple as possible.

In this scenario, I am a level 5 sorcerer and level 2 paladin.

Round one; cast haste on self.

Round two; try to hit target with longsword casting green flame blade, and expending a level 3 spell slot for divine smite, then follow up with a second attack using my 2nd lvl 3 level spell slot for a 2nd divine smite, but with my bonus action stop concentrating on haste, and cast thunderous smite as a bonus action, landing the hit for a total of: 1d8+2+4d8+1d6+1d8+2+4d8+2d6.

For the sake of this question, let's assume I have all of my spell slots unused.

Answer 1

Round 1 - Cast Haste - no issue, here. Though you could squeeze in one more attack (possible Divine Smite-ed) after casting.

Round 2.1 - Cast Green Flame Blade, on a hit, use Divine Smite - Divine Smite only needs you to hit with a melee weapon attack, which GFB does. No problem.

Round 2.2 - Hasted attack with Divine Smite - Pretty standard.

Round 2.3 - Cast Thunderous Smite and hit - Nope! You can only cast Thunderous Smite, but you can't attack anymore- having expended all your attacks. You already attacked with your Hasted attack. You can't Two-weapon fight either, because Thunderous Smite is a Bonus Action.

TL;DR- Everything is legal, but you can't attack after using Thunderous Smite.

Answer 2

No, this is not possible.

On round two, you don't need to use a bonus action to stop concentrating on haste, because dropping concentration is free. If you did stop concentrating on haste, you will lose this current turn and feel lethargic, unable to take the rest of your turn and your next turn per the spell description.

This means when you cast thunderous smite, you will be unable to take your actions to attack/cast spells. Thunderous smite only works as a buff, it doesn't let you also attack with the casting, so to use Thunderous smite, you would have to cast it before GFB/your 2nd attack.

Answer 3

Long Story Short: Not as such

Round 1: You cast haste. So far, so good, though I would then recommend using the extra action provided to make a weapon attack, boosted by Divine Smite if you wish.

Round 2a: Cast green-flame blade, using your Divine Smite for extra damage. So far, so good.

Round 2b: Extra attack from haste, Divine Smite for extra damage. Still working out fine.

Round 2c: Drop haste concentration to cast thunderous smite and attack. No can do. After dropping concentration on haste, the formerly hasted creature cannot move or take actions until after its next turn. While you can cast thunderous smite using a bonus action, you cannot make further attacks this turn, having consumed your extra hasted action, nor can you attack next round, due to the fallout from dropping haste. What this also means is that you cannot drop concentration in the middle and then cast/attack with thunderous smite, as the lack of actions start as soon as haste wears off.

Round 3: Can't do anything of note due to the haste fallout, though you may maintain concentration on thunderous smite should you choose.

Round 4: You can finally make an attack to release that thunderous smite you've been holding onto.

Tl;dr: Just keep haste up and either multi-attack or green-flame blade and haste attack each turn, it'll do more total damage. And it will also avoid wasting a 3rd level slot on what ends up being a 1-turn thing.

Answer 4

This would not work because the Haste spell when dropped leaves you temporarily exhausted until the end of your next round.

Now if you did the reverse:

1. Round one: TS -> GFB + DS
2. Round two: Haste, regular hit hasted action then Quickened Spell GFB

That would work.