@CaseySJ Hi and thanks for all your amazing help here!
I'm a few minutes away from building my system as detailed under my avatar and I have a related question to the above quote: while I don't have a second NVMe for Windows installation atm (because I frankly forgot to order one), I would like to have the option of installing Windows at a later date. Tonight I'll just be installing macOS (assuming I build this thing correctly and don't screw it up...it's been a few years since I've built a system).
Will installing the macOS drive in the lower slot now will allow for a smoother installation of Windows later once I have the second NVMe, or will I likely need to reinstall macOS after installing the Windows drive anyway? I suppose I'm not averse to doing that but my main concern is whether installing macOS in the lower slot with nothing in the upper would cause me problems now.
Thanks again for all your help - this guide is amazingly detailed and is very much appreciated.