Excel is known to kill Vals and possibly mosses depending on who you listen to.
Osmocote doesn't release over time when used in a tank environment. That isn't what is was created for, so they dissolve much quicker than they would if used for terrestrial plants.
If you want to dose lean EI, that's fine, but you'll need to keep you eye on your root feeders as they may or may not need some kind of root tab once a month to keep them happy. There are those around here that claim that only feeding the water column is necessary when it comes to root feeders; watch your plants. They will tell you if you if they need more food or not.
As for nitrates, the best way to get a feel for what your plants need and use is to test your tank for a few weeks. If you were to dose your tank at 30 ppm and nitrate levels are increasing before your weekly water change, clearly there is no need to continue dosing at 30 ppm as it isn't being used.
I don't dose the water column for nitrates in my 40 breeder. Between the bioload of my fish and what little nitrates are in the root tabs I use for my Vals, Giant Hairgrass, crypt and remaining small Amazon Sword, my nitrate levels stay between 10-20 ppm. 20 ppm is the high end which is at the end of the week when my tank is due for a water change. I'm currently using these https://www.aquariumcoop.com/collect...easy-root-tabs
and only 4 at a time. When the plants need more, I'll add them, but for now, plants are growing and happy with what they are being given.