Happy to help!
I don't think your tank is far off from stabilizing. You've had a bit of a rough start and getting a lot of mixed advice (I think I used recipe metaphor on the other site, but it's really more closely related to something like vegetable gardening. I think what you're looking for is like an organic garden with some composting
Imho, the substrate being disrupted when you refill the tank, and moving objects around is probably causing debris into the water which is then settling on things around the tank. That's what it looks like in this pic at least. Marimo balls collect debris a lot (and are a favorite of shrimps because of it), so to clean them it's easy to pull one out after a water change and gently squeeze them in the removed tank water.
Use the airline hose trick to suck out the excess debris and do the water change suggested on the other board. I bet after you lose the debris and stop kicking up new substrate, the ammonia level will drop back to 0. Your nitrates are good still.
I'm not sure what happened to your plants before, but its possible not all of them were low-light type plants, however, some plants do experience a "melting" when first transplanted. They don't die entirely, but do die back and then return healthy.
I think you're doing the right thing by stepping back to research more about the different methods available and decide what you want to do. Some people do just fine with low light plants under low light and occasional dosing of ferts with your substrate. (I believe root tabs can work in this situation)
You might consider posting a fts with a list of plants you currently have to get some feedback on whether they're good for your setup and suggestions. There are many low-light plant options.
btw, 15w for a 20g is less than 1 watt per gallon, but you do have the a good light spectrum. Low light tanks typically have between 1-2 wpg. Reducing the photoperiod, imho, was not the right direction to go with such a low light to begin with. When we know more about what plants you have now, it will help.