Whoah!
Finally, Orgzly (the Android app for org-mode) can sync with a local calendar, according to a recent changelog. THIS IS AWESOME. I have been hacking around with so many workarounds for so many years -- iirc:
1. emacs <-> org-caldav <-> RPi nextcloud <-> DavDroid
2. emacs <-> org-caldav <-> local radicale <-> Syncthing+DecSync <-> phone (current)
3. Plain org-agenda in MobileOrg (remember that?)
4. calfw (dead end)
5. Plain org-agenda in Termux
6. Going full paper (the dead tree) mode