Modern Android? More like Modern GNOME - it’s called adwaitapod and adwaita is the name of GNOME’s style. :-)

No, it’s just that when you use a mainline kernel, you’re just not reusing all the Android (often user-space) drivers that make cameras work on Android and due to that stuff, starting from drivers for the SoC camera interface to the camera sensor have to be re-implemented. Whether you are on glibc (e.g., on Debian/Mobian) or musl/Alpine does not really matter.

Also, Camera APIs and the whole “desktop Linux” camera stack (think of things like debayering, white-balance) is nowhere near as developed as what Android has (and that, IUC, Ubuntu Touch can reuse on Halium by plumbing things together).

A Pixel 3a may be a good choice. It’s older, but not huge—and it’s very well-supported in Ubuntu Touch (and Droidian, both use Halium/libhybris to re-use the Android kernel drivers), and also in postmarketOS (mainline Linux 6.9.3 as of this message).

On postmarketOS, camera support is not fully there—the front camera is somewhat supported. Also, Wi-Fi is still a bit annoying, calls only work with headset on postmarketOS, so I would say: Use Ubuntu Touch or Droidian for now, and maybe move on to postmarketOS once it’s a bit more solid.

It should be. With zram, I can use similar software even on a 2 GB RAM device somewhat comfortably, unless I open too many tabs in Firefox.

Very much not. GNOME Shell Mobile was funded by the German Prototype Fund in 2022 IIRC, way later than Phosh was created (funded by Purism for their Librem 5). GNOME Shell Mobile will eventually be part of GNOME proper (meaning it’s Mutter, and GNOME Shell, patched to work on small devices), currently it’s a patch set on top of multiple GNOME components that’s packaged in postmarketOS and the AUR (if you consider AUR stuff packaged).

Phosh was created on based on wlroots (which is also used in Sway and other wayland-native window managers) and GTK3, as a Mobile Shell. Ironically, this way was pursued because Purism developers where told by the GNOME Shell people that an adaptation of GNOME Shell for Mobile would not be feasible.

Both rely on designs created by (at least then) Purism-employed designer Tobias Bernard IIRC, and thus may seem quite similar despite being based on a different tech stack, and both are hosted on GNOME’s Gitlab, using all the same apps.