It can be done, but it’s complicated. You either need to mount the read only system as writeable (which will be overwritten with each update), or use a system like rwfus to install packages to user space instead.

You can also change your entire OS to Bazzite, which will keep the benefits of SteamOS (console like experience, suspend mid game, decky loader, etc) but has separate images for other DE’s like Gnome. I think the underlying OS is still immutable though, so if they don’t offer i3/awesome images I don’t think it’ll be easy to install them.