How do I set display scaling below 100%? That was the instinct but there is only a slider and it doesn't go below 100%, some googling got me some xrand commands for making a 1440p profile but that ends up with an out-of-range message on the monitor.

That might be Wayland-only. If you're on AMD, you can launch a Wayland session by selecting Plasma (Wayland) before you log in with your password. Bottom corner iirc. There are some drawbacks, but it's the future of the Linux graphics stack and where all new developments happen.

For Xorg, see "Scaling the desktop and simulate/render a new resolution" of https://unix.stackexchange.com/a/596888

For Wayland vs X, I'm not sure what this version is running but I'm hoping I don't have to think about that much. The main usage is just games which I got working in an hour tinkering and occasional vscode/ium and Arduino IDE.

You can ignore the gory details, but the gist of it is that Linux has used Xorg to drive the graphics for a good 3 (4?) decades, and was originally designed for terminals and mainframes so you could run applications on a remote server and render it locally. For perspective, for a while when you ran a 3D application, it straight up displayed the 3D box as an overlay on top of the desktop, so you couldn't overlap other applications over the 3D app because of the hack. Been solved for 2 decades, but that gives an idea of how much it's not made for that. As such, it's old and complicated and just not designed for how we use computers these days.

Wayland is the new one in development since the 2010s ish that aims to get everything right for the foreseeable future. But it still lacks some features, like until recently, no screen sharing and stuff. What it does however works really well. My Wayland session is way snappier than the Xorg one on 3 monitors. Worth a try, if you don't like it, just go back to Xorg.

Speaking of games, is it possible for Steam to consolidate its sandboxes? I imagine having so many redundant copies of mfc140 and other DLLs would be eating at the space? There also seem to be some effects missing, namely, Bloom. It seems to have issues rendering in comparison to Windows, is this a case of missing DLLs or just the proton layer not translating them yet?

I'm not 100% sure, but I think it's not a complete copy. A lot of those are just stubs so that they exist. On some filesystems, I think they're basically copy-on-write so they don't really take space even though they might show up as being large. The files only become copies when they get modified, so unless modified by an installer or whatever, they're the same file. Basically CoW is, you can copy a 10GB file 10 times over, it copies instantly because it doesn't actually do the copy, and although it measures as 100GB, it's only using the original 10GB. If you modify it, then only the changes take actual space. That might be btrfs/zfs exclusive though, my system has automatic deduplication and compression set up and I have more space than I can find uses for.

But otherwise, the copies have advantages in that one game may use one version, and another a different version that's not compatible, but it just works. Each game can get its own container with the ideal version of everything for it to work.

With the gamescope util, the %command% would be the path to the game exe/bin that's usually there?

Yes, but you also leave it as-is. Steam replaces it for you, so basically gamescope -w1280 -h720 -W1920 -H1080 %command% in the launch options would AMD FSR upscale the game from 720p to 1080p and Steam will fill in %command% with whatever it needs to run Proton and then the game in it. Just need to make sure the options are between gamescope and %command% for them to be interpreted correctly.