First of all, thank you for replying 💙 !

Can’t install a new system package for most immutable distros without going through some magic incantation

blendOS: Replace sudo pacman -Syu with system install

Fedora's 'immutable' distros: Replace sudo dnf install with rpm-ostree install

openSUSE's 'immutable' distros: Replace sudo zypper install with sudo transactional-update pkg install

While Guix and NixOS offer somewhat similar functionality with their guix install and nix-env -iA commands respectively, usage of said comments are rarely done by advanced users as other means to install packages are more sophisticated. And in terms of how sophisticated installing a mere package can get, one might argue that Guix and NixOS are to 'immutable' distros what Gentoo is to mutable distros.

And with that we just went over the 'immutable' distros that are prevalent in 95% of the discourse (besides Vanilla OS; but that one's in a major overhaul) and none of the commands found above strike me as particularly hard. Though, of course, your mileage may vary.

then doing a reboot

I'll just briefly mention that --apply-live exist for Fedora's immutable distros if you like living on the edge. Furthermore, both Guix and NixOS don't require a reboot in most cases. Finally, while the soft-reboot feature from systemd benefits all distros, one can't deny how impactful it is to 'immutable' distros in particular.