I use arch on my main machine and since steamOS is based on arch, that’ll probably be your best bet for getting all the stuff you need to work.
But besides that, what’s wrong with steamOS? Is using flatpaks instead of a classic package manager really that much of a dealbreaker for you? If not, just keep using it and save yourself the hassle of maintaining another OS.
As a desktop? The immutable OS is a pretty damn close to insurmountable limitation. Flatpaks are fine for casual stuff, but they're pretty limited overall. Not having the option to genuinely install real software is an issue basically the minute you go from dedicated gaming device to full time PC.
Most things, if not available as flatpak, can be installed inside another distro on distrobox. It runs in containers, so things can access a root filesystem (Just not the main SteamOS one), and is a pretty seamless experience, once installed. I have a bunch of non-flatpak software running that way, and it works great.
What are you trying to do that doesn’t work with flatpaks? The only issue I ever had was that VSCodium would throw random “not enough space” errors even though there definitely was enough space.