Duuuude. I just wish I saw your comment BEFORE spinning and fighting with a nextcloud container. Well...At least I didn't go all the way in just yet. Just found out ICSx5 does exactly this (it popped when searching for icsdroid on f-droid). My calendar is populated with the Proton Calendar. For my use, I can create events with proton calendar, and Android gets it to the local calendar via ICSx5. Thanks man!