The steam deck uses the desktop environment called KDE Plasma if I remember correctly. I recommend using the Fedora KDE spin since, right after Debian, most apps will support Fedora. It is user friendly, feature rich, stable, secure (with massive community and corporate backing for timely Security updates), and simple.

Dual booting is a smart decision. If you opt to dual-boot, I recommend encrypting your system through the built-in OS installer. This stops Windows (or malicious software) from spying on your new install. It is also just a good idea in general.

Pop!OS doesn’t yet support Wayland, which supersedes the old and slow X11 with better security (on X11, any app can capture what you type, their is no isolation).

Bazzite seems neat but I wouldn’t go for a gaming focused distros in my experience.

Canonical, Ubuntu’s parent company, is trying hard to create a closed ecosystem. Even though Ubuntu is Debian based, they are making it hard to install native applications, instead enforcing the use of Snap, which uses a closed-source backend to provide the app repository. Snaps are also slower than native or Flatpak apps.

If you need any help, explanations, suggestions, or other thoughts about Linux, I am willing to help best I can or point you in the right direction. Ive installed linux maybe 50+ times on most of the major families of distros (Debian, Ubuntu, Fedora, openSUSE, Arch Linux, and some others), and have personally used many distributions that are derivative of these. I’m not like crazy experienced, just familiar (with a focus on Security).