Wait what is the difference compared to the previous behavior?

Afaik Firefox has nothing to do with your compose key input. Are you sure this is happening only in Firefox? Of so an explanation might be that you have a font rendering issue, and the symbol the compose key produces is different in your system font compared to the font Firefox uses? Can't really explain how you'd get different symbols in apps otherwise.

Did you read the notes about the Nvidia proprietary driver? If you want HW acceleration with Nvidia, you need to use a translation layer to VAAPI using CUDA (see the notes on the pages you linked). Be aware that it might not work due to, well, Nvidia being Nvidia. But it should lead you in the right direction if you haven't done that already.

Plex is absolutely not "designed to run on Windows". Both systems work absolutely great and in any case, a Linux server is a lot more stable than a windows server. Plex is also very lightweight on resources (when not software transcoding), so it won't affect your ability to run other stuff on the same machine. Only managing a single server is going to be a lot easier for you as well.

A piece of advice on matrix. Don't run synapse on the same server, use either dendrite or conduit as synapse is a bit of a resource hog, while the other two are nicer to your cpu and should work well for small installations.

I love the gnome workflow. Coming from MacOS it's more familiar to me than a windows layout, but still so much better than macOSs defaults. I usually have 3-4 workspaces open, with a specific "environment" in a single or a few workspaces. E.g a browser window with email, todos, calendar etc and other "personal things" in one, maybe one for a certain project I'm working on, another for a work project, etc. This way I'm always focusing on one thing at a time but can quickly context switch and have my laptop "switch with me". I also make heavy use of alt-tab and Ctrl-tab for window switching. Together with fewer windows per workspace, this makes it super fast to navigate without ever taking my hands off the keyboard. If I forget where things are, a glance at the overview is enough.

It should be noted that I don't use a mouse and if I love touchpad gestures, so gnome is perfect for me. Even using a keyboard only and the very occasional touchpad is very comfortable on gnome. At least compared to macOS and windows.

I highly doubt this would affect Fedora. Thankfully, it's community driven and self-goverened so Red Hat execs can't go and tell them what to do. (Though I don't know how many ties the Fedora council had to Red Hat)