I agree with you that there may be unknown additional side effects, even at the relatively low levels that are used to protect teeth, especially children's teeth, from cavities and toothy decay.

It's unfortunate that that line of inquiry has more often than not been bad-faith reactionary chud shit, like antivax movements are and with similar overlap. Some vaccines do have known side effects, but it's messy to even talk about it because of the chud factor.