You can install and use a standard distribution through Distrobox, which has a few initial steps to install, but once it’s there, you can dnf/apt/zypper/apk/pacman/etc. install any packages completely in user space. (And distrobox-export from within any Distrobox containers lets you access a command or graphical application from that Distrobox transparently without having to manually run a distrobox command.)
And distrobox-export from within any Distrobox container sets it up so can you access a command or graphical application from that Distrobox transparently without having to manually run a distrobox command.
But, you probably want to not have anything private on your Steam Deck, as there’s no encryption, so anyone can take your Deck and copy files off of it. To solve this, you’d want to either have an encrypted loopback file that you use like a pluggable disk (there are some tools for this; “Plasma Vault” is even built into the KDE desktop that powers the Steam Deck’s desktop mode, but requires a little setup) or you could run a Linux distro (or other OS) within a VM in GNOME Boxes (available as a Flatpak without having to do anything special). The Plasma Vault solution is more lightweight and integrated, once you install the encryption support it needs.
FWIW: I used my Steam Deck as a desktop when traveling a few months ago and have been using Linux (with GNOME, not KDE — but both are great) for decades.