If you’re using Evolution from Flathub, not working in F31 vs. F32 could be due to a mismatch in the evolution-data-server API from host to the flatpak. (However, I think the Evolution flatpak also ships its own evolution-data-server as a fallback, or something wacky like that.)
GNOME Tweaks isn’t out of favor. The extensions have just been moved to their own app. Everything else in Tweaks is still fine. Keep happily using it for all the additional tweakables. 
When I’ve had RPM Fusion installed before, I had to do an rpm-ostree reset to remove the distro and overlay packages before doing a rebase… and then re-added. Agreed about it being a bit of a pain. At least RPM Fusion is becoming less needed over time?
As this is my work computer, I’m not so fussed about the lack of exFAT support. And the Flatpak version of official Firefox (on the Flathub beta channel), Kdenlive, and VLC and such all have proper codec support, so it isn’t a big deal to me.
On my personal laptop, however, exFAT support is critical for using some of my SD cards with my camera (which reformats SDXC cards to exFAT), so I’m also looking forward to the new kernel that will bring full exFAT support soon. A work-around is to reformat the SDXC cards to FAT32 ahead of time, as the camera still supports that too, but it’s something you have to think about and get right beforehand. I have RPM Fusion installed on it for that reason. (But my personal laptop isn’t using Silverblue, just “normal” Fedora 31, so it’s not as much of an issue there.)