What if I added a homebrew action called "Concentrate" where if you spend your action, you get advantage on the con save to maintain concentration on a spell if you take damage before your next turn?

Rule 1: Must have concentration spell active before you take this action.

Rule 2: Effect ends if you cast a spell (i.e. saying command word of magic item is okay, but casting a spell with bonus action, reaction, etc. ends the effect)

Rule 3: If you take damage on your next turn before declaring the action again, you don't get advantage (i.e. if you use your movement to provoke opportunity attack before declaring Concentrate action, and you take dmg, make a con save with no advantage)

Rule 4: Cannot use this and use a spell states you use your action (or possibly bonus action) to interact with it (like shooting a beam of light from Wall of Light).

(please rip apart my pitch and find loopholes, I would value any and all feedback)

Example: You and a fighter want to kill a guard in a small room, while not alerting the guards outside. Roll initiative, your roll is higher than the guard's. You cast Silence, and the fighter runs up and starts wacking the guard in your room. That's the surprise round. On your turn, you take Concentrate action on your turn. The guard to yell for help, cant, and attacks the spellcaster in a desperate bid to stop the Silence and call for help. Guard attacks, crits for 30 damage. You roll a 14 on your con save, normally a fail (30 rounded down is DC 15). Second roll success though, Silence remains in effect. Fighter takes their chance and kills that guard, your Silence spell stayed up, no one heard a thing.

Thoughts?

Edit 1: clarity

Edit 2: added rule 4

Edit 3: clarity, grammar, and corrected the example's surprise round rules

Warcaster?

Quote from FriendlyHeathen >>

What if I added a homebrew action called "Concentrate" where if you spend your action, you get advantage on the con save to maintain concentration on a spell if you take damage before your next turn?

Rule 1: Must have concentration spell active before you take this action.

Rule 2: Effect ends if you cast a spell (i.e. saying command word of magic item is okay, but casting a spell with bonus action, reaction, etc. ends the effect)

Rule 3: If you take damage on your next turn before declaring the action again, you don't get advantage (i.e. if you use your movement to provoke opportunity attack before declaring Concentrate action, and you take dmg, make a con save with no advantage)

Rule 4: Cannot use this and use a spell states you use your action (or possibly bonus action) to interact with it (like shooting a beam of light from Wall of Light).

(please rip apart my pitch and find loopholes, I would value any and all feedback)

Example: You and a fighter want to kill a guard in a small room, while not alerting the guards outside. Roll initiative, your roll is higher than the guard's. You cast Silence, and the fighter runs up and starts wacking the guard in your room. That's the surprise round. On your turn, you take Concentrate action on your turn. The guard to yell for help, cant, and attacks the spellcaster in a desperate bid to stop the Silence and call for help. Guard attacks, crits for 30 damage. You roll a 14 on your con save, normally a fail (30 rounded down is DC 15). Second roll success though, Silence remains in effect. Fighter takes their chance and kills that guard, your Silence spell stayed up, no one heard a thing.

It's a bit complicated and interacts poorly with War Caster.    This is what I would suggest:

Meditate:

When you take the Meditate Action you are more focused on whatever you are concentrating on. Until the start of your next turn, you gain +5 to any Constitution saving throw to maintain your concentration.   Taking any Bonus Action or Reaction ends your Meditation.

Try this cantrip:

Determined Concentration - Spells - Homebrew - D&D Beyond
https://www.dndbeyond.com/spells/597296-determined-concentration

Hardly fair to that poor guard in the room.

I think it is best to leave it be. Concentration is intended as a powerful counterbalance to some spells. In some ways it might be the only reasonable way to defend against an ongoing spell if Dispel Magic is not available. Remember that Concentration is used to balance some big spells that can range from giving your party auto-crits against enemies to making you immune to all damage. It's there as a way for non-spellcasting enemies to break the spell effect.

Giving advantage to the checks greatly steps on the toes of Warcaster, giving a bonus can mean it gets out of hand for a Bladesinger that already gets a +Int bonus to their concentration checks.

Perhaps instead of something they can just immediately do, make it a Feat with a 8th level prerequisite, have it add a set bonus equal to their proficiency bonus. Essentially this would mean if they're proficiency in Con Saves they're now expertise where Concentration is concerned. This will enable the bonus, not interfere with Warcaster, doesn't immediately start at max and gets better as you go, and the level prereq means they're not going to be cheesing it right off the starting line.

Also remember: what you can pick and when you can pick it, remains true for the enemies as well. Would you want to face an enemy Bladesinger upcasting their Hold Person to paralyse most of your party while it's lackeys wail on them - and the one or two who weren't paralysed having a hard time breaking that paralysis because the Bladesinger had +10 to the save with advantage?

I'm not saying don't have something. Just think it is better to be cautious about when and how they can get it.

Or keep it as some freebie but then ensure to re-adjust every possible encounter you'll throw at them in order to compensate. Personally seems like a lot of work for something not needed.