I have also been trying to set up my google calendar. Default google calendar works just fine, events are synchronized to both directions. Shared calendars are not visible at all in phone. I removed google and caldav (owncloud) accounts couple of times and removed manually related .db files. After recreating accounts, I still see only default calendars. Android tablet sees shared calendars, so I think calendar setup on server side should be ok.

Another issue is that after adding calendar sync to google account, Jolla calendar app lost previously working caldav calendar. I had to remove and recreate my caldav account to fix it.