I think this topic is - kind-of xy-problem.
The real problem is not having a custom data folder - but rather keeping PostgreSQL at a specific version.
When postgresql is updated from one major to the next, there is update procedures that must be run to ensure the database structure matches the new major version.
While I have little experience with PostgreSQL - I know that systemd overrides created to change the default behavior of a service will always survive an update - suffice your overrides are created in /etc/systemd/system and this is important - **never touch the files in /usr tree - your change will be overwritten with an update.
If you want to ensure a postgresql production database is never touched by an update, add postgresql to the list of IgnorePkg in the [options] section of /etc/pacman.conf.