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.
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!