The closest thing to SteamOS is ChimeraOS. Though it sounds like you're mostly referring to Plasma, which is the desktop environment (DE) AKA the graphical user interface (GUI), and can be installed with just about any distro. ChimeraOS looks like it only comes with GNOME, which I personally prefer anyway.

It is very friendly until you need to do something other than very basic things, at which point you'd better be prepared to become very familiar with the terminal.

Bazzite is the new hotness though, and it's what I use, because it comes with a variety of customizations out of the box, and the ujust commands can greatly simplify a lot of common tasks that are otherwise far more complicated. Only downside of that is that it mostly only supports flatpaks and appimages, .deb and .rpm are the most common package formats and those have to be installed in containers, which comes with all the fun complications of containerization and sandboxing.

You might give GNOME a try though if you want to play around with something different. I much prefer it from an aesthetic perspective. Plasma is essentially made to look like Windows and GNOME more resembles MacOS.

Be aware that when using these distros that some games will recognize your device as a Steam Deck and apply some fucked up graphics tweaks that severely limit performance. You can undo them with a simple launch argument but it's super frustrating until you figure that out. Protondb is your friend there.