IMHO you should first figure out what exactly happens/goes wrong with your Ubuntu installations.
Immutable distros might or might not be a solution, but if the core of the problem is really the quality of the Ubuntu updates for example, you could try to run Debian (stable).
But again, the suggestion to use Debian is throwing a solution in the room which might not fit your problem.
Just as a reference point: I am running Debian stable on Laptops, Netbooks, Raspberry Pis and in virtual machines (AMD64/AArch64) and have no weird bugs, everything works for years now and runs smooth.
Concerning the Steamdeck… I love them, they run perfectly fine, but unless you are tweaking them/do more than run games, you cannot really compare them to what happens on your desktop.
It sounds really strange, that you end up with the problems you described given your usage.
My systems are heavily modified/tweaked, so one would expect I would experience the problems you describe.
Given your usage, using an immutable distro sounds like a no-brainer to me, immutable Linux was created with your usage scenarios in mind.
In your shoes I would still try to pin point the root cause of the error, because in theory™ your usage should not be a problem for any of the mainstream Linux distros and we don’t know if an immutable distro solves your trouble.
Given your 6 montish circle it sounds like some kind of accumulation? If the computer runs stable for several month, IMHO you can rule out hardware problems, unless you have a kernel update every 6 months… :-P
Can you be more specific about your hardware, laptop model and Ubuntu version you are using?
If you ever figure out what happened, or if you try out an immutable distro and it runs for a year for you, give us an update! :-)