I have jumped the gun on thinking that the template as a file was working. It's not. It's the same story: the initial file template is good, but it's for sure getting cached somewhere, and an updated file is ignored. The same as when the template was being drawn from the database.
Are you just updating the file /opt/mailman/mm/var/templates/lists/testlist.example.com/en/list:user:notice:welcome.txt with nothing in Postorius and either no entry at all in the mailman.template table with
name = 'list:user:notice:welcome' context = 'testlist.example.com'
or if there is such an entry, with
uri = 'file:///opt/mailman/mm/var/templates/lists/testlist.example.com/en/list:user:notice:welcome.txt'
If so, editing the file /opt/mailman/mm/var/templates/lists/testlist.example.com/en/list:user:notice:welcome.txt should work.
Progress!
It now works if the template information is removed from BOTH database tables (mailmanweb.postorius_emailtemplate and mailman.templates).
Just removing the row from the "mailmanweb.postorius_emailtemplate" table and changing the URI to "file:///opt/mailman/mm/var/templates/lists/testlist.example.com/en/list:user:notice:welcome.txt" in mailman.templates didn't help.
So now, with both databases cleared, mailman looks for the updated file without any problems.
Thank you very much Mark, once again, for your solid advice and patience.
Best, Mark