some writer snob somewhere: Do not start sentences with But or And because doing so is grammatically incorrect.

me, writing my fic: But I don’t care. And you can’t stop me.

Sometimes…..the flow of your creative prose…..is more important….than grammatical guidelines,,,,,,

That isn’t technically grammatically correct. This is a rule made up by elementary school teachers to keep small children from writing in fragments. And I, for one, will always be a pedantic grammar nerd.

Also, there is no rule against splitting infinitives, and singular “they” has been part of standard English usage for centuries.

Your teachers were full of lies.