I (finally?) succeeded in converting the main site (server.)erbenet.nl to Drupal 9 as well.
This one was a bit more complicated than the previous one (websites.sillybutterfly.nl). After all, SBW already ran on the Bartik theme under Drupal 8, and that was easy to transfer. But SBGS ran under the Mayo theme. That is not (yet?) available under Drupal 9, unfortunately. Converting to Bartik turned out to be more complex than expected and I don't like the result... But it worked!