1. Multiplayer games are difficult to make, especially if they're real-time. I struggled making even a simple turn-based game with all the orders of events I had to address. It's also difficult to debug.
2. This it quite an ambitious project. Make a prototype first. Without importing levels, boss fights, etc. See if you like it.
3. Menu controls are a bit weird, better to support arrows maybe just in case? Or make UI similar to the map layout.
4. OK now I see your plan, "bare bones" should not include settings, why start with the boring part? Again, see if you like the main idea first. Adaptation to the monitors should also come a bit later (you don't even have the full UI yet!)
Overall, being a fan of Osu, Thumper, having played FNF and CotND, I am not sold on this idea. Too many buttons for me, the lanes layout is weird. Nothing setting it apart. But as a project to learn game dev it should do just fine.