Debian Testing + flatpak
Testing is shockingly stable, kind of up to date, and rolling. Since you will use Flatpak for all your apps it really removes a lot of risk that dependencies will break an app.
I use this combo as my daily driver for my work PC, knock on wood it's been super solid!
Have you checked out OpenSUSE MicroOS at all? It's similar to Silverblue. Runs great on my Framework 13 with Intel 13th Gen.
I wouldn't be too concerned with "officially" supported Linux on the Framework. It is a very Linux friendly machine. The folks they have supporting Linux are active in the Framework forums and very helpful.
I eventually went back to my tried and true Debain. I loved the immutable OS thing for all the reasons people have listed here. My one issue was direct access to external devices can be a pain. IE: I just could not get USB passthrough working with virt-viewer after all my fiddiling.
You're missing out on chasing the dragon for the latest and greatest. :)
Arch is fine once you get it setup, but I feel like the nerd in us can never just leave it be. I'll probably go back to pop_os next major release they have.
After trying i3 and sway for a bit I've landed on just using Forge and Gnome. I really would recommend trying it. It's my daily driver for work.
It's a fully dynamic tiling solution and on top of a traditional DE.
To clarify, Cosmic desktop is not the default. It's very much a WIP. Pop OS uses Gnome by default. They add some nice customizations to it too like tiling support and some enhanced power management options.
Pop OS is Ubuntu based, but they replace Snap with Flatpak, package a kernel as close to mainline as possible, and include Nvidia drivers (if you grab the Nvidia installer ISO).
I used Pop for a few years, loved it. Last I used it they still defaulted to Xorg instead of Wayland and that was a no go for me with an eGPU so I switched to Opensuse.
"If Books Could Kill", they cover the airport books that captured our hearts and ruined our minds.
One of my new favorites.
I tried HoloISO and had pretty mixed results. I've had much better luck with ChimeraOS.
The devs on ChimeraOS are excellent too, they take in community feedback and are very helpful.
I would take a look at pop_os. It's Ubuntu, but without Snap and a closer to mainline kernel version. They have a lot of great usability tweaks too.
I run Arch BTW. I just like to make things difficult :)