I'd go with the EI system, too.
I do check for nitrate, ammonia, nitrite, KH and pH, but this is for the fish not the plants. For the plants, I check KH and pH, but that is only to keep the CO2 level constant since I inject CO2. In a low tech tank, you don't need to check these.
After a couple of years of testing nitrates, K, PO4 after and before every water change, I can say that for the plants, if you are using EI, the levels you have, even if you have very accurate measurements, won't change very much.
Essentially, if everything is looking good, you don't need to test.
Since you have Discus, I might check your tap water for nitrates and ammonia, and make sure that you don't have lots of CO2 in your tap water, or low O2. But that is only if you're not going to age your water. If you age your water for 24 hours, only check for nitrates. Some people have very high nitrates in their tap water, and nitrates over 40 ppm are definitely not good for Discus. They can get that high with EI if you already have lots of nitrate in your tap water.