I love my Niri/Noctalia setup on my laptop.
So I thought it could be a good replacement to KDE in desktop mode on the steam deck. Use one of the touchpads to scoll the tiles and desktops.

Has anyone tried it?

I haven't tried that exactly, but I put OpenSUSE Tumbleweed on mine. I think installing a mainline distribution is the first step you have to overcome to run a different desktop environment on the Deck.

The biggest hurdle will be that you will need custom drivers or maybe even a custom kernel. Because not all necessary drivers have been upstreamed yet. This is especially necessary on the OLED version, as far as I know.

If you go that route https://gitlab.com/evlaV will be your best friend to find everything you need. And Bazzite probably as well. Maybe you can even run Niri/Noctalia on Bazzite. That would probably be the easiest. Or maybe CachyOS has a Steam Deck build.

Zirconium is a ublue image for Niri desktop, just like bazzite, but it doesn’t contain gaming specific stuff. The maintainers there were working on a bazzite+zirconium image called zirconium-jackrabbit but it seems defunct now. Discord for the project is here: https://discord.gg/2sj4JbbqK

Ok, but wouldn't it be possible to just install niri on the existing OS and change the "exit to desktop" mode to launch niri instead? Gaming in niri wouldn't be that important while in niri because I mainly to that from the Steam GUI.

Or is the steam GUI reliant tied to KDE in some way?

If I recall correctly the script that goes to desktop mode explicitly starts plasma. Though I think they want to change that behaviour in SteamOS 3.8 or 3.9 to be more generic.

The trouble is that on every update all the changes you did to system files would be removed.

So you'd have to install Niri to your home directory and change the scripts that launch it with every update. Nothing impossible just a little bit annoying.

That would be a little bit annoying. Maybe a script that changes the script then :)