I do think its sensory and we've had the same issues with our son. Now that he is 12 we don't pin him down anymore, of course - just lots of lecturing
We've been lucky in that our son happens to have a decent balance of bacteria and enamel that seem to keep cavities from forming. But he is at severe risk for gum disease that will cause him MORE problems than cavities in the long run, and he knows this. The dentist diagnosed him with gingivitis at his last visit, and that knowledge managed to improve his brushing efforts for a few weeks.
Eventually your daughter will have to take responsibility for her own oral health - or lack of it - on her own. But until she's old enough, I don't see that you have a whole lot of alternatives to the current method. I hated it to, and I think it carries side effects we may not even know, but sometimes there are no good choices.
Still, keep looking for them. We did have some temporary success with a couple of tricks, so I'll describe them, but they always wore off
One is using a timer. It gives them something to focus on and watch, and a sense of how long to brush.
Another is to sing and brush together. I turned a Wiggles song into a brushing song: "you can, move your hips and brush your teeth ... brush the very back, brush behind the teeth, brush the big grin spots, get everywhere there could be rot! You can, move your hips and brush your teeth."
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).