I'm going to guess (but I'm not an expert on active substrates) that your Eco Complete is adding some N and the Osmocote is certainly adding some. How much? You'll have to look into that. You don't need root tabs (Osmocote). All nutrients can be supplied in plentiful quantity via the water column.
The 50% weekly water change is designed to keep EI dosing in check, but does not account for non-dosed nutrients (Osmocote and, maybe, Eco Complete). Plus, fish are going to add to NO3 as well. No one knows for sure what plant uptake rates are, but they are estimated to be between 3 and 10 ppm daily for NO3, depending upon many variables. The best thing to do is to test, like you are, for NO3 and PO4. If you follow those numbers obver time, you will get a feel for how much is needed weekly. Based upon your statements, you can probably stop dosing NO3 and see how the readings go. NO3 levels are considered acceptable between 10-60 ppm for high-tech setups. I prefer to see levels in the 20-30 ppm area. Check your PO4 levels before deciding whether or not to cut back on it. Keep it in the 3-5 ppm area. You can't measure K, but it is good to keep that in the 20-30 ppm to match the nitrates.
BTW: By "drops" I assume you mean a drop of the reagent for the test kit. So, 6 "drops" = 6 degrees. Let's talk in that nomenclature. for KH, 6 dKH (degrees KH) is higher than I like, but acceptable and, since it matches the 6 dGH (moderately hard water), it's ok. The Eco Complete is doing most of that. When your CO2 drives the pH below 7, as the acidity increqases it dissolves the carbonates in the Eco Complete and they raise your KH and GH.