[Solved] After trying several troubleshoots this finally helped: go to settings > system > language > regional preferences and set everything to standard. Then force stop the Google and Gboard apps, clear cache and data on both apps. Then activate and set up the Google assistant. Then wait a few minutes as files are being downloaded. Then Assistant Voice Typing should be available.

If you mean the language packs in the Google app, then yes I already tried that.

Thanks! In the meantime I also found a video by In Depth Tech Reviews (great channel) in which many changes are shown.

Is there a comprehensive list of all (user-facing) changes and new settings?

It changes size randomly, and also slightly moves upwards and downwards from time to time to prevent burn-in. This is totally normal.