Many if not most studies don’t clearly demarcate pre-omicron covid vs omicron. I’d love to know what the science is on myocarditis risk from just omicron is.
The decrease in serious illness is due to prior immunity and vaccination.
Though it’s obviously a hard thing to prove either way. But certainly I do not think it should be assumed that omicron is inherently more mild.
Here is recent, high quality one, but there are many more (and some with conflicting results - but to my eye the best studies all point to similar conclusions).