When using a smite spell cast on a previous turn, can you use your bonus action to apply another smite spell to the same attack?

Question

Essentially this comes down to an order of operations question when it comes to concentration, in Dungeons & Dragons 5th edition.

Say you have a smite spell cast on the previous turn, and you are concentrating on it. If you cast another spell that requires concentration right now, the smite spell will end. You have both an action and a bonus action ready this turn.

You attack and land the hit; you are now ready to apply damage from your weapon and your smite spell.

Are you still concentrating on your smite spell at this point in the attack sequence?

If you are not concentrating on your smite spell anymore, can you then use your bonus action to cast another smite spell and apply it to the attack before resolving the sequence of the attack, making a series of events that look like the following, assuming enough spell slots?

Example: a paladin

• previous turn:
  cast thunderous smite, gain concentration
• this turn:
  Declare attack, starting your attack sequence.
• Succeed in hitting your target
• Apply thunderous smite to your weapon attack, lose concentration
• Apply Divine Smite
• Use bonus action to cast thunderous smite, gain concentration
• Apply thunderous smite to your weapon attack, lose concentration.
• Roll damage for your weapon attack, your thunderous smite, your Divine Smite, and another thunderous smite.

Answer 1

This does not work

1. Thunderous smite triggers when you hit somebody:

   The thunderous smite spell states:

   The first time you hit with a melee weapon attack during this spell's duration [...]

   In your scenario you have already hit the enemy and are then casting a second thunderous smite, this would not do what you want because thunderous smite only deals its additional damage when you hit, and you have not actually hit any enemy while this thunderous smite was active (being concentrated on). You hit before casting thunderous smite.

2. You cannot take a bonus action in-between hitting your opponent and dealing damage to them

   This is rather intuitive. If I hit you, I cannot run away or cast a spell or do all sorts of other things before you actually take that damage. What allows a bonus action to come between hitting your opponent and dealing damage but doesn't allow for movement or simply more attacks? This is also gone over in the section "Making An Attack"

   You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise. [...]

   On a hit, you immediately roll damage, you do not get to cast a spell in-between these steps of making an attack.

   This is covered to a certain extent in the following question:

   • Are getting hit and taking damage simultaneous events?

3. Ending concentration on thunderous smite means it won't deal damage

Ending concentration on a spell ends any and all of its effects, this means that even if you are able to hit the enemy and then stop concentrating on your first casting of thunderous smite you would not deal the extra damage.

Note that you don't actually lose concentration on thunderous smite when hitting an enemy, you still have to end concentration yourself, or have it end due to taking damage or casting another spell. You could continue concentrating on it after already applying its damage there is simply very little reason to do so as it only has an effect the first time you hit somebody, though there are some reasons.

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That all said the other parts of your scenario do work, you are capable of combining Divine Smite with a smite spell as gone over (somewhat) in the following question:

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