Install and use sysusers.d/tmpfiles.d config files

Install sysusers.d and tmpfiles.d and use them. Stop manually calling adduser. This also allows image-based systems to be created with /usr/ only, and also allows for factory resetting a system and recreating /etc/ on boot.

https://www.freedesktop.org/software/systemd/man/latest/sysusers.d.html

Also stop deleting users/groups on remove/purge, as that is considered bad practice, as any potential leftover file/directory can then become owned by the next user/group that gets added, with unpredictable consequences.