Regarding Ubuntu 20.04 Gnome and Mate desktop versions. I've downloaded the .iso files, copied it to USB stick, booted the USB stick and performed a fresh installation to a HDD. This seems to provide stable versions.

If you have "Notify me of a new Ubuntu version" set to "For long-term support versions", then you may be wondering why you haven't been notified of the 20.04 release as yet. My understanding is that around June or July there will be release 20.04.1. At this point you will receive the notification. You will then be able to perform an on-line update from either the 18.04 or the 19.10 release to 20.04.

There are methods of "tricking" the 18.04 or 19.10 versions to do an online update now, before waiting for the 20.04.1 release. From my recent attempts with Ubuntu-Mate 18.04 to 20.04 the online upgrade it will fail. With workarounds you can complete the upgrade, but you end up with a rather buggy and unpredictable system. So I recommend that you wait a couple of months for 20.04.1.

cheers,