Except for the pre-flop sizing, you played it right. Make it 50-54. If you know he never folds, just make it bigger.

Raising the flop accomplishes very little. He folds all his random bluffs, and he doesn't fold better. Only reasonable hand he continues with is AQ. He might take this line with AJ and 88, which are reasonable hands for him in this spot.
I'd write something about protecting and balancing your range, but I'd just be talking out of my ass.

Clear call on the turn. Unlikely he has diamonds, and how did the 5 possibly help him? The sizing feels fairly weak.

You could fold the river, but I'd probably call. There are only a few value hands for him, and you'd have to think he check-raises them on early streets, instead of double half pot donking.
He might see the third diamond as a scarecard if he's bluffing, and he might be scared of it himself if he does have a hand that beat you on the flop, so him betting bigger on the river than on earlier streets wouldn't make sense.

Btw, do players who are this bad still exist at 3/6?!

I like the sizing preflop and flop and turn are pretty standard, imho. On the river I had folded. You´re certainly very close to the bottom of your range and not exploitable if you fold AK in this spot.

I agree though, that his sizing is very weird, so calling for the price you get and get to see what the hell he´s doing is very likely not a big mistake at all.

OK, the preflop size might actually be a bit too small, I had made it like 46-48, but no way I made it 50-54, there´s no need to bet 3x or even bigger in position.

If you exclude AJ from your preflop 3-betting-range you´re right, then it´s getting thin, but I went with the assumption that we have ...

If you want to extract more value you want to make it bigger preflop, especially given your read that he is loose and 'donkish'. Given the river sizing how is bet/pot ratio increases (from half pot half pot) on what would likely be a bad card for him and improve your range. I'd fold and feel pretty good about it. You could make an argument for raising the turn, though I'd guess calling is fine as well. I'd probably want a few more reads before deciding to raise the turn for value or let him barrel his air or worse value hands that he may fold on the turn. In a vaccuum I guess I'd just call the turn.