I wasn't really worried by the river so much. There's perhaps 4 combos of flushes in villians range given all the blockers that are out there,

,

,

and

(maybe).
In my experience, randoms (and he was random to me at the time) might take this line with these hands up to the river, but then they just donk jam. So when he checks it's a sort of green light that he doesn't have a flush. Of course it's possible he'll play these hands that way, but given that there's only 3 or 4 combos of them anyway, I decided it wasn't worth worrying too much about the flush.
That isn't to say the river is a brick - it massively reduces my perceived bluff combos, thus making it less likely for villain to hero call with the hands we are trying to get heroed from which are: AA, AK, KQ, AT, JT, J9. There's still a lot of combos of these hands (massively outweighing the combos of flushes, so even if he folds these hands 80% of the time and calls with a flush 100% of the time it'll be a fistpump jam combinatorially (forgetting the full house combos for the time being)).
The turn is the most worrying card. When he checks it, it's another "green light" that we are winning. Probably. It is an unusual line to check a fullhouse on the turn in a 4way pot with all the draws / hands that can call but will check back / scare cards on the river. It's still a worry though, more of a worry than the flush imo. And there's way more combos of houses/quads than flushes (13 combos)
Will he call with worse? Random frenchy - yes, quite a lot of the time. I've seen them call with KJ here, snap AA, KQ and I think most would struggle to fold Tx or J9.
That's why I jammed.
He had QQ. I've since learnt that he's a good but slightly nitty reg. Do we like his check on the turn? Now we know he's a good but slightly nitty reg, do we prefer check back, 5k/fold, or jam?