> To say the biggest source is Gmail might be technically wrong though - I suspect there's a large volume of spam that Migadu (my provider) is dropping before it even reaches my inbox, i.e. emails that it is 100% sure are spam and it can just drop. Nevertheless, an overwhelming amount of spam I observe/have to deal with is coming from Gmail. Second to that is outlook/hotmail.

This. It's more likely to be survivorship bias -- the gmail emails happen to survive because gmail is more trusted.