abirdcall
(she/her)
I have given my reasoning in several posts.
So let's go over this specific instance:
This spell allows for a better chance of banishment to be successful by forcing disadvantage on the saving throw. How much better? If I remember the math it gives banishment about a 15-25% better chance to succeed.
That's pretty good, but it's not certain, it just gives a better chance of success - at the cost of a reaction and a spell slot in addition to the banishment slot.
It does not overcome legendary resistance and it means the caster won't get to use shield, endure elements, counterspell etc.
Also it is "good" because banishment is good (though I personally think banishment, while good, is overrated) meaning it has to be part of a combo to be truly effective - that lessens it's stature.
Another way to look at it's impact (you did, after all, call it the best spell currently in the game) - does it have the potential to win an encounter(like sleep at lower levels, like hypnotic pattern, like banishment on a solo monster)? By itself, clearly not. Paired with a spell like banishment? It will aid but not ensure.
With all that - yes, seems like it will be strong, but I'm still not seeing overpowered.
If the creature saves against banishment it is the same as casting banishment again only for just a reaction and a 1st level slot.
That is absurdly broken.