If you are comfortable tinkering a bit, I’d recommend checking out the Decky loaderThis tool adds the ability to get a lot of different plugins for your steam deck, from changing the home theme, to editing fan controls. There are a lot of tools here.
If you want to emulate, emudeck is another great tool. It’ll set up a bunch of stuff for you and download emulators and set them up with good configurations for steam deck. I also believe if you do this, it will set up decky loader for you (don’t quote me on that, it’s been a while since I set it up)
For epic game store games, id recommend checking out Heroic launcher. It’s an open source launcher that connects to gog and epic games accounts and lets you download and play games you own on those stores. Another option is The non steam launcher tool on GitHub. This allows downloading a bunch of various launchers, including epic.
As for Xbox game pass, the situation is less ideal. If you have gamepass ultimate, you can utilize Xbox cloud gaming to play your gamepass games. This article is by Microsoft to set up cloud gaming on your deck. If you want to play then natively, you’ll need to dual boot your steam deck to have a windows partition. I have personally never done this, but I have friends who have, and it works well for them.