Mavic Does Solus make it easy to install the latest nVidia drivers? I didn't have to worry about this on my Thinkpad.
Yes. We have a hardware drivers tool so NVIDIA is basically a 2 (or 3 click, if you want the 32bit NVIDIA stuff for gaming too) process. Just please make sure you do a full system upgrade and reboot first.
Will I run into any issues setting up PlayOnLinux/Wine/Proton?
Can't speak for PoL and most of my Windows gaming experience (including with Proton) on Solus occur via Lutris, so I'd suggest using that.
Does Spotify show up on the sidebar widget?
Yes. Anything which exposes itself over MPRIS shows up in Raven.
Has anyone tested libratbag & Piper using Solus for their gaming mouse?
As the maintainer of it on Solus and owner of a Logitech G403, yes.