Is there a clean way to manually import Contacts etc. via PhpMyAdmin?

Yes and no …

In PhpMyAdmin you can go to your Mautic database and select the table Lead then do a PhpMyAdmin export using CSV but it’ll export everything including anonymous contacts if I recalled correctly into CVS so it not exactly “clean”.

What I would do is a SQL export in PhpMyAdmin then do a fresh Mautic install on another location with a new database then do a SQL import into the fresh installation …