Am I killing my basil?

I wouldn't even say I'm new to hydroponics, I'm actively still trying to figure out how it works. Only just found out about needing to keep some roots in air so they can breathe, used to keep them topped up.

TLDR: what can I do to help my plants, and what will hurt? for a total newbie

problem: recently leaves have begun to turn almost white at the bottom, or get brown patches or dry up and just fall off. I was worried about root rot first, and I definitely think that's what happened with the darker shade on the two biggest ones, BUT there's still healthy white roots in there, if you look at the closeups. should I cut the bottoms off entirely and just start again? this was before I knew they needed air.

more history: these basil have a much longer and more frustrating story, after fighting mold while sprouting, taking months before they got an inch or two tall and realizing they needed more light, to root rot because I was too slow on changing their soil to a better draining thing than they sprouted in, to the most recent problem, fungus gnats. where, and I'd never had them before but one other plant started this whole mess (quarantine your new plants… for more than a month) but after finding fungus gnats even looking sideways at these I said NOPE and just cut them all off and propagated them. there's more smaller plants, doing well not shown here.

basically, I've fought hard, and I desperately want a basil bush… I am really just trying my best. light is an issue too, this shelf in a south facing window (in MO) is the best I've got right now.