• Self-hosted Radicale for calendar, contacts, and todo list
• On Desktop use vdirsyncer for syncing and khal/khard/todoman
• On Android I use davsync with mostly default android apps for calendar, contacts and todolist
  
  Works flawlessly.
  
  Use custom curl scripts to get some internet calendars that also works flawlessly.
  
  Been doing this for almost a decade now.

Irrelevant. You can sublicense MIT to GPL by forking if you’re so inclined.

I have been using Zotero for a while and syncing my library directory with Syncthing, even though they say not to (no problem in 7 years including PhD and job).
If you want something even more minimalist, it is possible with the command line too, , which is the approximate setup I’m currently using in my pharma job.

I would vote for syncthing as it can have better support if you need syncing across work firewalls. Also allows device-to-device sync, not just server-device. It’s a cool federated solution (like lemmy!).