@Roger, @agriesser: Thanks a lot for your help and your valuable input.
I just performed the migration and Mattermost still seems to work! 
@Roger: One question which does not become clear from your message (or I’m missing it constantly):
You also installed the exactly same MM version on both machines and let the new instance initialize the PostgreSQL DB schema - right?
I had a little trouble after running pgloader - Mattermost insisted on re-running the last DM migration 92 “add_createat_to_teamembers”, or at least it wanted to insert this value again into the db_migrations table even though it already was there.
This caused an SQL exception (duplicate index values in unique index) and the start to fail.
It worked after I manually deleted the offending existing entry (DELETE FROM db_migrations WHERE version=92;), even though I’m not happy with such manual DB interventions and I don’t understand why Mattermost insisted on re-running the migration even though it was already entered in this table.
I can only assume it happened because I had already run into MariaDB incompatibilities with Mattermost 7.3, and e.g. the Focalboard plugin could not execute it’s DB migration, causing the plugin to be forced disabled. Maybe due to this, the last migration somehow was not logged / marked as “completed successfully” - but that’s pure speculation on my part now.
Nevertheless, after removing the offending entry, the server logs didn’t seem to show any other unexpected error messages, the server came up, Teams, Users, Channels and messages are there, embedded images show - so looks fine for now, as far as I can tell…