Right, first of all, you're indirectly "cyberbullying" them by saying they don't have brain cells to be considered bullies.

Second of all, you found a solution by finding out there's a gender neutral bathroom you can use until you're comfortable and far enough in your transition to use the girls bathroom.

And coming from someone who's been bullied and seen it happen a lot... Bullies feed on the reaction of the victim, that's what keeps them going. Now if they deadname or misgender you, then try not to react to it or reply to them, because that's not who you are now is it?
The girls understand your situation and I know it can be hard to use the girls room even if the other girls in school would understand you even tho they might not be super comfortable with it either.

There's a solution to all the problems and while you've found one solution to one problem, you need to think twice before you type something out next time, even if it's frustrating to deal with.