My website refused to upgrade beyond 4.4.9. The reason? The database was mysql v5.7.42, and Joomla 5 requires mysql v8.
The solution was to create a new database, then export the "old" content and import it into the new database. This was easily achieved using phpmyadmin. The only problem was, a couple of reference to the old database name (and possibly database user) existed and needed to be edited manually in the exported sql file. But that's ASCII test and a simple text editor is sufficient.
The database then needs to be referenced correctly. the server settings (system > global configuration> server) have all the things that need to be changed: database name, host name, database username and password. The database type (host: Ionos) is set to mySQLi.
With the database manually updated, the upgrade to Joomla 5.2.0 then sailed through!