NixOS is semi-immutable but not really designed to be user friendly. I think we are more talking about Universal Blue, Fedora SilberBlur, OpenSUSE microOS, VanillaOS and so on.
Tbh I'm pretty new to nixos, but I'm starting to believe if we had that the exact same config (at least without flakes, I'm still having trouble undertanding them) but with a slightly abstracted UI, it would be one of the most user-friendly distros out there.
Like just imagine being able to click "Add program", write the name of a program, having all the options appear below as dropdown menus or on/off switches, then click big blue button "Apply" to rebuild.
As it says on the website, this is still in development and not actually ready for use by mere mortals quite yet. It hopefully will be at some point though as that is its explicit goal.
Apart from the game mode update notes being hard coded to show steamos updates, bazzite is a drop in replacement that doesn't get rid of your non flatpak packages each update. It also bundles or has easy installation options for all the recommended third party software everyone uses with the deck.