Since you are already familiar with the Steam Deck, I would go with a distro (that's Linux-speak for version) that uses KDE Plasma, as that's the desktop environment used by Steam OS. My recommendation therefore is Bazzite or Aurora. Both OSs are developed by the same team using the same core technology. Bazzite is more "gamer focused" (a.k.a. they are pre-installing steam) while Aurora targets more of a developer audience. But I would argue that the differences are, at the end of the day, mainly cosmetic. Aurora is still pretty noob friendly and you could still develop on Bazzite.
Both systems are "Atomic" which basically means that a system update can't screw up your system. And if you screw something up, then you can "rebase" the system which reinstalls the OS but keeps all your data and installed apps*. It uses flatpak with flathub as its appstore so you have the same apps available as on the Steam Deck.
Now for the downsides: Some apps are a bit harder to install. Mainly apps that need deeper system access such as vpn apps can take a couple more steps to install here than on other distros. There is also no live system. Many distros have a "live mode" where you can test the system before installing. This allows you to check if your hardware is supported. Aurora and bazzite don't have that.
Lastly the userbase is growing, but other disros such as Mint are still more popular. You therefore might find less specific documentation and tutorials on the internet for it than with other options.
*Technically it's more complicated than that, there are edgecases of apps that don't survive a rebase, but don't worry about that.
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Both apps work for both platforms