I assume Pop!_OS shows the Systemd-boot selection at boot. If so, is it possible to switch which partition to boot to?

It uses the Arch official repositories so it's as rolling as pure Arch, i used it before it's great. It uses KDE Plasma and has different theming options to pick from that all look really nice.

You can always try it in a live environment before installing it to see if you like it.

If i'm correct proton adds a lot of gaming specific patches that increases game compatibility fixes in steam. Outside steam i've been using wine-ge which i find better than normal wine because it adds the proton patches and more which you can read about in the wine-ge-custom github.