I've read all kinds of information online ranging from underwatering...overwatering .. mildew ? bacteria? disease ... remove the infected leaves, more sun ... I really have no idea how to narrow it down.
You read too much.
Personally I don't feel like I'm watering it enough, but the soil seems constantly pretty damp and water runs right through.
There's your issue right there, you see the problem but are too inexperienced to realize it.
I have grown many peppers in various size pots but always 1 per pot, see pic below.
I will offer some advice on growing a plant centered in a 5 gal. pail... Put some kind of tall stake in the center, I use some salvaged trunk springs from my automotive days, as this will serve two purposes. The first is is to attach the handle to it so as the plant grows it grows around it otherwise the plant Edit: soddenly suddenly prevents the bail from being centered for lifting bucket. The other is, unless you cage, if the branches start to sag due to fruit weight you have a method to support them.
See the bail near the bottom of the pic and how the plant grew around it?