That is a strawman for decriminalizing drugs. When you don't wear a seatbelt, you risk others' financial harm. We deal with that largely without loss of other people's lives. When you mug people for cash at knife point to buy heroin, peole get physically hurt. You cannot possibly sign a waiver saying that you will not mug, steal, break-in, etc. if you decide to try heroin.
Mind you I am not arguing against decriminalization or legalization of drugs. Just saying that your argument is bogus.
Second, given that most people have insurance, and that the ones who don't are free riding on emergency care for any injury, but yet you're saying that you would support deillegalization of non-seatbelt wearing if one were to sign a specific waiver (which the cartel of insurance companies would then prohibit you from signing anyway, no doubt), I don't believe your argument to be sincere. I can only infer that your real viewpoint is that universal mandatory seatbelt usage is a good thing, and you're just making yet another specious justification for dictating individuals' behavior.
FWIW, I've held up starting to drive for people who weren't putting a seat belt on, and I think it's pretty ridiculous to not wear one. I just don't think the government has any business mandating such behavior.
On top of the above, I spent many years in a country with a poor healthcare system and lax seatbelt laws. The results were not pretty.
Lastly, in your original post you seem to suggest a possibility that the government telling you to put on a seatbelt is somehow comparable to being a slave in the early days of this country. If that really was your suggestion, I am not sure how to respond to such misrepresentation of reality.
Drug prohibition is actually based on tax laws. The bulk of federal drug deillegalization will not even take an act of congress - they'll just start issuing tax stamps.
Seat belt laws are based on the idea that government/society has a financial interest in the productivity of your well functioning body. This analysis essentially treats us all as mild slaves. The conditions are certainly better than the racial slavery of yesteryear (although I'm just repeating the groupthinkline here; I don't have a time machine), but the ideology is not.
Pooling money for common purposes and avoiding free riders has a moral leg to stand on. However, the current system has massively outgrown this philosophical justification, using the sheer majority of revenue collected for maladaptive antifeatures.