Thank you to @kjathome and @SchBenedikt for your patience and assistance.
So, this is really crazy…
I’m not sure if one or more of the suggestions above fixed the problem, or if the problem “just fixed itself” somehow… but magically my issue has been resolved.
The problems went away after flashing a backup image of my server that i’d made with dd in case I made things worse. I had basically given up solving the issue after trying everything above and other things i could think of.
What’s crazy is that the dd image of the server was made with the issue unresolved, yet flashing it “fixed” the problem.
At this point I’m not going to question it and just be happy that everything works again.
I thank you all for your patience and assistance in this matter!
TL,DR
Summary
I had tried all the suggestions above, multiple reboots, combinations of the two, issueing the above database commands, messing with permissions in the commandline via SSH, and nothing worked.
I had given up on ever solving this and decided to start over from scratch, reinstalling NextcloudPi and just loading my data from a backup. Before doing so I manually backed up all my user data and made an image of the enire server using the dd command, just in case.
As luck would have it, starting over did not go well as a myriad of other problems (all of which would require their own threads) prevented me from getting up and running again as a new instance.
Finally, after days of torubleshooting I gave up again and just flashed the dd image of my old server (with the issue that started this thread unresolved) which I decided I would just use without the contact functionality…and somehow everything just works!
The contacts app had an address book already there, despite multiple failed prior attempts to make a new one, even after the above commands were issued. But now I can create and delete contacts with no problems.
A separate issue with the webGUI displaying all white with scrolling disabled similar to this issue has also “fixed itself”.