I managed to get Widgets and Search working again with the new database backend.
This also means that the changes from FocusGTD2 for widget support on Oreo and higher now also works as expected (see previous blog post about all the pain, blood, sweat and tears).
So overall this is the status:
Will GTD3 be a paid update / new version? Or will it just be an update to FocusGTD that I have already purchased?
FocusGTD3 will be a paid upgrade. I have been working on it for over a year.