You have better hands to defend here. He doesnt really rep any flushes so your As blocker is irrelevant. I would prefer to call with a Q, blocking Q9. But it seems to me that when he overbets a river that is not good for him to bluff at, he should be pretty strong and his overbet is actually his way to get you into a call. Also, if he has 55 or 99 here, he knows you will have a hard time folding a Q here and that you might have some flushes too. I would fold this particular combo and might even fold strong hands here if this villain does not take lines like this often

"I float this because the only value hands I see him having is 55, Q9 and the rare 99 (expect him to 3bet this from time)." You c-bet 1/3 pot, so you better balance that with KQ + otherwise your hand will always be air, super marginal and villain can raise QT+ for value, especially when you start calling down with A high. I would just have folded earlier, x/call flop or c-bet a bit more OTF.

I think if I were to cbet this flop which I probably would, I would cbet 1/2 pot.I think calling 99 is very standard, but I don't play 100NL so I could be wrong however I don't expect BB to 3bet 99 vs MP open because 99 plays well in single raised pots so he can have 99 here. I think he can have AQs here some percentage of the time as well. He doesn't have a lot of value combos however we only have ace high and I think he would raise his value hands more often then not on this board texture.

Overall I think I would just fold to the flop raise without any reads, I don't think this is a spot where we need to be really sticky. If he's bluffing he's got a lot of equity because most of the time his pair outs are live along with his gutter/OESD, and if he's got 2 pair + we are dead, so I can't see a winning situation for us. If he's got 2 pair + we are going to lose all the time and if he's bluffing he's gonna win half the time and the other half we might even just get bluffed off because we have just have ace high. And also if he has 2 pair+ if we spike a pair we might go broke.

Aight, thanks guys for responses, really appreciated and helpful :)

I get that cbetting small is the new thing these days, but the point of it is so we can cb extremely wide on certain textures. This is not one of those textures. I would generally be checking a wide portion of my range here and when I do cb, cb larger. Just something to add.

I'm folding flop but as played, I suspect if villain is decent he recognizes this is a bad river to bluff so is trying to maximize with his small value range. That is more likely than he is on some sick level knowing you will interpret it in this way.