Given the code is flakey and unfixable, would it be an option to replace the current code with a whole new, off-the-shelf engine that works better? That might reduce the load on your shoulders by giving you better control over what you can allow Users/admins to do. Kinda in the vein of what Apple did with they stopped developing their OG system and switched it all over to a rebranded NeXT. Though i realize this place is no Apple.

If that does pique your interest, i found this list of free forum engines that work on Linux: https://itsfoss.com/open-source-forum-software/