Unless there are major changes from the Deck version, I would STRONGLY advise not running SteamOS on any machine with sensitive data. You want a real login screen and, preferably, FDE for any laptop.

If you like the Desktop Mode? That is just KDE Plasma. Basically every major distro has a build that uses it (and you can install it yourself otherwise). And Steam mode is literally just Steam Big Picture (with some minor tweaks).

For a laptop? I am an old so I use Fedora. But I think everyone loves atomic distros these days so consider Bazzite.