pre mastery level update doing monstrosity was what got me from 7-8, and when that happened all i did in that particular session was get Bugard from like 35-46. now whether that means I unlocked x- number of instincts, or x number of exp/lvls in monstrosity, i don't know, but SOME monexp seems to count.
Now, I do have a feeling that some of this content has one "tier" of contribution towards mastery, and in monstrosity's case its relatively low. I hit rank 9 with nowhere NEAR capped monexp, my highest monster is 70, and i have loads of species to unlock. I focused on the ones that required trades to unlock in case they counted as quests.
Rank 20 Wing skill seems to be the notable contribution tier for digging, for example. (i got it to 100 anyway, still rank 9) and MMM I don't have the 500/1000 titles, so maybe it's the 100 title or the runes themselves that is "the tier"
none of this is fact of course, just observations from being on this mastery rank journey from day 1. I'm still going at these for completion in case i see 10, but i get the feeling we will start seeing them once the new HTBF etc come out this year, or when people gather enough trust points.