Sam,
First and foremost, welcome! From one former manhattan resident to a current, I know your pain!!!! Most people on here won't bat an eye at your apt. comment, however the struggle is real!!! Lol, I at one time had 3 tanks going in a one bedroom!
I, much like you forayed from African's to discus... Funny thing is now I am on long island, nice sized home and I have one tank at work, lol, go figure!
Anyway... you seem to be part of a growing trend here. Educated people doing the right thing and researching here first before they dive! I was not one of them and it was a struggle, and still is sometimes because I can never seem to stay out of my own way. I guess it wouldn't be a hobby if we didn't tinker right???
Anyway, I am happy to give my suggestions and help based on what you wrote and I am sure others will chime in to help you.
I think you are well on your way to being successful with a few minor tweaks....
First off, good move stepping up from the start tot he 75, another thing I wish I did. You will be very happy you did it. Does your building ever have power outages or surges? I lost an entire tank of gorgeous show Africans after a weekend outage when away and my canister's didn't fire back. I have always had a dislike and a battle with canisters. I honestly prefer the ease of a nice HOB over them. My fave though is a sump, so you may want to consider it.
Based on your desire for some light sand, wood connected plants, tank mates and all, I would do your best to up those water changes and keep above 50% at least until you see how it fairs. I would also strongly suggest bigger fish to start and make the investment to start, because again, given your desired regiment and the feedings and work it takes to grow discus and the amount of growing left to do at that size I would suggest going big. Just like you don't want to see fry being eaten, you also wont want to see stunted or iffy discus in the tank.
Also, given your lack of qt and all as described, you may want to choose a supplier who offers mates, wither already with discus or on the same system or something like that, or be aware of the potential risk of mixing and adding fish. You do have some good choices locally to select from, may even be fun and worth it to go visit these places or hand pick. I have yet to have the pleasure to visit hans, but I have been to discus maddness and going to any fish importing setup is always fun!