You say the villain is unknown, but before our first decision we learn loooads about him:
He’s a passive optimistic fish. He’ll call you down with all sorts so bluffing in general is a bad idea against him...
Having said that, in this example I think we might want to raise it up against him preflop. Nice and big. In a small pot a player like that is never folding but if we make ourselves look real strong we can make redline profit off him. If we keep applying heavy pressure to these passive fish they get used to it too, and then they start calling you down in huge pots with terrible hands.
I do like your preflop check. It is probably a good idea to wait until we see a couple more hands against this guy before we start running crazy bluffs with 53o.
So now we’re on the flop. We have two options: Try make this guy fold... or... Sit back and wait for a hand.
Playing any draws real passively (by just checking/calling until you hit a hand) is always a valid option against a passive player. This guy could have TPTK and just check back the flop! If he’s handing out free cards like that we’d be a fool not to take them.
Alternatively you can try to make this guy fold. He has a lot of air in his range so this is a perfectly valid option too.
These are the two main strategies to choose from but in reality all we actually get to choose is whether to bet or check.
If we lead out with a bet, like you did, we are trying to make the villain fold with what is going to be a very small bet against a guy who don’t like to fold.
If we check, we are fulfilling both our objectives: If the villain checks back we take a passive line and sit back. If he bets, we can raise over the top with a far scarier bet than the donk, a bet that makes more profit than the donk, gives us more information and builds the pot bigger for when we do hit. The raise here is way better than the donk for making redline profit.
Leading out with a bet here doesn’t fulfill any of our objectives. It’s just a bit, meh. You’d expect this passive dude to call you down all day long so by all means do it with a weak pair. You could donk pot with a strong pair. But don’t donk in a limp pot against a passive fish when you have a gutshot.
Donks looks weak, generally speaking.
Then the turn comes, changes everything, but you stayed the course. This makes alarm bells ring! If the board ever changes heavily and the villain continues with the same aggressive strategy he’s probably still trying to bluff you.. You gotta switch up your strategy on the J because if you are repping strength you are repping a different hand to what you had a second ago. You could check raise, you could check and then lead the river, you could change your betsize, anything except continuing on with the same sized barrel...
You carried on with another barrel and now your hand looks even more like air than it did when you donked the flop.
On the dry river card your range is so weak this guy could call you with K high.