I kind of hate to fold the nut flush here but I don't see them doing this with anything worse that we beat. Kcxx does not play like this, most likely up against a boat. I like your line up until the river, where I'd fold.

bet larger otf vs fish, bet turn, call river

i think we have to call this, doe his line screams boat. But if they don't x/r their sets otf here, they also can do other strange things otr, in many cases they donk worse fd otr here.

Yeah, bet flop much larger. Turn aswell.
OTR: mehhhh, 55/15 does seem passive, but I can't fold vs an 80bb fishy player. They can also shove some weird 7 or even 89 or QJ here.

You should definitely bet bigger on the flop and turn.

As played, it's really hard to say what's optimal with the limited information that you provided because this can be a call vs. some recreational players but as a general rule I wouldn't make a habit of calling overbets from passive players. Also, with Q and J on the board, there's not that many worse flushes villain can be doing this with.