1- Did you see this issue on Github? https://github.com/linuxmint/nemo/issues/3270 - maybe there is a solution in there.

2- How did you install Steam? On the official forums lots of people reported problems when having it installed via Flatpak or Mint system package. Best practice is to install it via the .deb file from https://store.steampowered.com/about/.

Maybe you're new enough that you haven't found the Mint forums,

https://forums.linuxmint.com/

the best source for fixing obscure Mint problems, and full of Cinnamon users -and- a decent search. I only wonder how many people use Nemo from the command-line.

Can you share your "uname -a" and what filesystem did you install (ext4) ?

It sounds to me that your installation was not properly done and/or you have some permission problems.

Whatever it is a re-install would be the best solution.

Yeah, I've had some issues with Nemo as well. I ended up installing Thunar and have had a much better experience with it. And you can set it to be your default file manager if you like it.

Getting games to work can be a little tricky, depending on the game. Be sure to check out protondb for comments that others have. Sometimes it's just a matter of changing the version of proton. Not sure about the other usability issues you're describing.

Do you have an Nvidia graphics card? If you do, and if you haven't already, you'll probably want to use the proprietary driver instead of the open source one (which is selected by default). That might even help some of the Steam issues.

I also get the error message "Could not register nemo as a service, trying as a remote: Unable to acquire bus name 'org.Nemo'" just before the message in the post

Have you tried disabling GPU hardware acceleration for Steam? It sounds like steam is rendering poorly - I had a problem with blacked out pages with steam with nvidia and found it resolved been I turned off GPU acceleration. This was a while ago though - I'm not having issues on latest drivers and have GPU acceleration back up.

Here is a guide on how to turn it on/off within steam itself. https://thegeekpage.com/how-to-enable-disable-gpu-accelerated-rendering-in-steam/ Note this is purely for the steam client, it has no impact on games or proton etc. The intro on the linked page seemingly misunderstands the settings it then talks about!

In terms of Nemo, I've not had that issue myself; on searching I can see random different solutions to the problem so it doesn't seem simple to resolve. Although frustrating you could switch to a different file manager. Thunar isn't bad and works well with cinnamon. PcMan-fm also isn't bad. There is little difference from Nemo with either. You can find both easily in the software store, or in Synaptic or in a terminal via searching with apt. Once insralled you can then change the default file manager in the cinnamon settings.

My favourite is Dolphin but I use KDE rather than Cinnamon. You might not want to mix KDE and Cinnamon apps? (It'll work fine, but they do have different design philosophies and aesthetics, and may be jarring when compared to other file dialogues within other apps).

Update: I've moved back to windows for now as there were too many problems (more than I listed in the post) and not enough time to fix them. I'll give linux another try when I have more time