Hi e_riddle,
Thanks for using Apple Support Communities. We understand that notifications aren't working as expected on your Mac after recently updating macOS. You've don a great job trying to get this resolved. We're happy to look into this with you.
We recommend that you test the issue while in safe mode. Starting in safe mode allows your Mac to run certain checks, remove caches, and prevent login items from automatically launching. This can isolate possible third party issues. Use the steps and information in the link below for assistance with safe mode:
How to use safe mode on your Mac
Also, does the issue occur in other user accounts? If you don't have another user account, use the steps in the following resource to make an account and test the issue. This will help us determine if the issue is user specific or system wide. Set up users, guests, and groups on Mac
Let us know the results. Be sure to include any other troubleshooting steps that you may have tried.
Have a great day!