I do not like the immutable distro approach at all. In fact, it is one of the several things I do not like about Android. I want to be able to install the software I want, to upgrade or downgrade packages individually if needed, etc.
The astOS project you link to seems to actually implement some compromise, where whatever is deployed is still built on the user’s machine from packages, but in a snapshot that is deployed only if and after upgrading succeeded. Note that this still uses pacman, so I doubt that is what “moving away from pacman” would be referring to. As far as I can tell without having actually tried it, that approach sounds more flexible than centrally prebuilt “take it or leave it” images, but that flexibility comes at a performance and disk space cost that I doubt will be appreciated on the PinePhone. It also locks you into btrfs. The PinePhone images currently use ext4.
From my experience, the main reliability issue with upgrading the PinePhone is actually not pacman at all. It is the PackageKit pacman backend, which is awfully buggy. Upgrading with Discover has caused all sorts of breakage (see Discover system updates still very unreliable). If I use pacman directly from the CLI, it works much better. That is what urgently needs to be fixed to make upgrading work for average users who do not use a CLI (especially on a device with usually no physical keyboard). Replacing the whole pacman (and likely the whole concept of packages, because I doubt it would make sense to use, e.g., RPM or dpkg instead of pacman) is a sledgehammer approach that is likely to just introduce more problems.