Human ashes could progressively become more carcinogenic because of our bodies getting more and more filled with microplastics.

I doubt it, because the plastics would almost completely burn off, and while the concentration might increase I don't think it would ever be enough to leave behind substantial residue for quite some time. Plus most of the time ashed are kept in urns or distributed in places that already will have microplastics and other carcinogens

Nah, not the cremains but the smoke contains all the volatiles and is not insignificantly toxic.