Keep changing it up :) It's normal as they are tired and not that cooperative at the end of the day and it's not really fun or interesting to brush your teeth is it? They don't understand the longer term consequences so you just have to keep holding firm about it.
If you have different ways to make it more fun then you can give her a choice - shall we sing a song today or look for the colours? Do you want to use the Paw Patrol toothbrush or the Elsa one? Strawberry toothpaste or spearmint? Often in supermarkets you can find all kinds of different toothpaste to try. Sometimes the same flavour with a different character or picture on it seems different to them too.
Google Home has a tooth brushing song DH found which my kids love even though I think it's a bit naff. Lots on youtube as well with every character from Blippi (though, don't let him in if you haven't already!!) to Peppa etc.
I used to get DS1 to "sing" using any tune you like, first using the sound eee eee eee eee, then using the sound aaaa aaaa aaa aaaa - that gives you the right mouth shape to access the fronts of the teeth and then the tops and backs of them. I called that "Eeee and Aaaah song".
Take it in turns as well - let her brush and then you have a go, or you go first and she goes second, or she "brushes" yours while you brush hers.
Try something silly like upside down teeth brushing where you lie her back over your lap and do them. Or backwards tooth brushing where you walk into the bathroom backwards and have a drink of water first and then brush them in the opposite order that you normally would.
We also used to do games where I'd look for "colours" in their teeth and then guess all kinds of unlikely things they had eaten. Green - have you been eating grass and crocodiles?? Oh, broccolli, oh that must be it. I've heard some people look for "animals" and then make the animal sound when they "find" the animal.
DH sometimes pretends to be a robot with a speed control so they can make him go slower or faster by turning a pretend dial. Then after they got used to that game the "dial" gets broken and he's an out of control robot going fast when they ask for slow etc.
All little ways to give them control and make it more interesting help. If she genuinely seems to struggle with the sensation of it, giving her control like controlling the speed or letting her try different kinds of toothpaste or brush can also help.
We recently put a smaller mirror on the wall next to the bath, and my 3yo has been absolutely fascinated by the fact that he can see what he is doing to his own face (one day he was really chocolatey in the bath so I gave him a flannel and showed him how to look that he was wiping the chocolate off) - it made me realise that with the depth of the sink and all the hand soap, taps, toothbrush etc clutter there, they couldn't really see themselves in the sink mirror, so a lower down one they can get really close to has been a hit here.