I also wouldn't bother with the new substrate for the plants, unless you're wanting a color change. Root tabs work fine. And then the water column fertilizer for each water change like Flourish.
I think you need to look at the minerals etc in your water to see what you're missing (sounds like potassium).
It takes a while to figure out what plants work for your water/light. Once you get your water / fertilizer schedule set, then the ones that die don't work.
Keep trying. Some will work better than others.
My fish are used to a densely planted tank with a lot of cover and much dimmer light due to the dense nature and the floating plants. I think they'll settle in more with your plans of bulking up decor/plants. Even more wood will help. You can then tie on anubias which are typically low maintenance and don't need to be in the substrate.
I tied a piece of my wood to the top of my tank so it did't cover the substrate but still provided cover - you could try something like that. You want to still be able to see the fish.