~850 effective...main villain is new to the table, limp calls a lot, seems like a 1/3 reg/fishy reg

Main villain limps UTG, MP raises to 25, Hero flats AhJd on button. Main villain cals.

Flop (75) AcAs7d

Checks through.

Turn (75) 4s

Checks to hero. Hero bets 25. Only main villain calls.

River (125) Qc

Main villain donks 100. Hero raises to 350. Villain snap shoves all in. ~500 for hero to call.

Is this always a boat? Bluffs that made sense to me were 56s, busted hearts. Are we “underrepped” having checked the flop? Does it matter that we ubnblock hearts? Value range I thought he could maybe play like this were 44, 77, AQ,A7o, A4o (but is he really limp calling A4o and A7o) and is he really checking Aq/AK twice?

To the bolded: Yes. Yes he is.

Being "underrepped" is meaningless against a weak & passive player. They're on level 0 thinking (what do I have, what is the betting action?), and you're trying be on level 2 (what does my opponent think I have?). Trying to be 2+ levels above your opponent is wasted mental energy, and likely to cost you.

On the river, your opponent is overwhelmingly likely to only call with hands that are beaten by your AJ. His potential bluffs, if they exist, would snap fold. Even a hand like 77/44 may only flat call your big raise out of fear. Though they're rare, I think you get shown aces full here if you call. So fold.

I find a lot of your plays in this hand questionable though. All 3 options are viable preflop, depending on reads on MP. Flop check is OK I guess, but I lean towards a small bet there. After checking back flop, I prefer a larger turn bet ($40-50). Then when V wakes up with a large bet on the river, I think we should strongly consider only calling. When blatantly passive villains "wake up" with a very large bet on a later street, it's quite often a very strong hand.

Just flat river. You're never getting called by worse.

3 bet or fold pre.

$40 on the flop.

$55 on the turn.

River I agree with just calling considering the size of his bet. I guess we puke fold as played, but in game I honestly probably call considering villain is a fish and he might be inadvertently bluffing, thinking an A is good.

Flatting pre is okay.

I bet flop, bet bigger turn, and only flat river. Never raising here and if I do it might be to $200 or get a call from Ax, pp, KQ or something - also gives opportunity to fold without puking too much. Imo you went too big here.

You're always beat to his all-in - as played I likely puke fold. Calling is burning money.

Seems like an annoying, but standard fold.

Bet the flop.