Thanks, that advice helps! Do you have any specific recommendations based on the pictures I posted as to why my plants aren't doing well with NPK, Flourish, and glutural? I feel like it's lighting at this point but any more light leads to bad algae issues for me. I might get a BGA product but I feel like that's a band aid for an underlying problem.
I agree with previous comments that you are probably deficient in nitrogen. The plants you are keeping are well within the range of what can be kept in an inert-substrate low tech tank fed with fish mulm, so dosing nitrogen isn't necessarily required. However, if you are having trouble balancing your bioload with the need to do water changes, then perhaps a little nitrogen supplement might be helpful.
I can't tell super well from the pic, but it seems the abubias you are growing as an epiphyte (cannot take advantage of substrate nutrients) is doing fine, and only the plants with the capacity of getting nutrients from the substrate are in need of assistance. That suggests to me that you could solve this problem while taking advantage of the benefits of substrate nutrients mentioned in my previous post. Root tabs that provide nitrogen might be easier for you than frequent liquid dosing.
It is my personal view that dirted tanks provide more benefits than an inert low tech setup, but with roughly the same maintenance costs; therefor, if you are going to go low tech, you might as well go dirt. I understand a full conversion would be a big hassle, but it is something to consider. You would not have to worry about nutrient issues like this (for the plants you are having trouble with).
Anoter DIY trick is to mix (mineralized) dirt with water, put in a cube tray and freeze. Next you take the cubes and push them deep beneath the substrate. When the plants hit that region with their roots, you will notice.
I missed dukydaf's entire post on my first read of the thread. My bad! Just wanted to add that I support the above advice. Considering the size of your gravel, I would only add that you might want to pour some sand over the part of your tank you are dirt seeding to ensure you have a strong cap over the dirt.