Hi Jakes,
welcome to Simply. Nice looking setup you have there.
I’m not a big plant guy, but I’m not sure the Echinodorus actually require any kind of fertilizers. Omitting those would be beneficial for the development of your discus, considering the still have a lot of growing to do.
Your target stock looks reasonable, how many fish your tank can support will mostly depend on filtration and maintenance. It will come down to your preference, I’ve seen people keep 8 or more fully grown discus in this size tank, with 6 being the absolute minimum as they feel more secure in numbers and to avoid too much aggression towards one particular fish. There will always be an “ omega”, but more fish will distribute aggression a bit more equally.
There is a wealth of information to be found in each sub forum, especially in the stickies, so I recommend you browse through those and read up on what applies to your plans and setup.
So enjoy your stay here, keep us updated and best of luck with your endeavor. With the right care discus are hardy fish and not as hard to keep as it is made out to be at times