Hi Aidan,
I am on holidays for a few days with a limited access to internet, but my quick reply to
"Do you have further plans for extending RaiseWikibase?" is yes. I'll try to
handle with those secondary tables.
Regarding (2): can that be done in MariaDB?
If you can make (3) done, the whole Wikibase community would be very thankful.
Regarding (4): yeah, too radical and maintenance problems.
Sorry for a short answer, I'll add on my return.
Kind regards,
Renat