Thanks for your comments.

There is a GitLab instance with a repository which holds some of the public code:
https://gitlab.anhonesthost.com/HPR/HPR_Public_Code

Your suggestion for  the transition from the old to a new tag system is pretty much what I had in mind, but we haven't yet discussed all the issues amongst the Admins.

The site is not static, though there have been discussions about making it so. I take your point that there's a conflict between having a static site and offering tag query features though. 