The open calendar, task and note space is a mess
Surprised nobody has mentioned this yet, but Org-mode for Emacs [1] is just great, and fits very well to the requirements:- Source of truth: these are text files, so any of git, nextcloud, syncthing etc. will do.- Consistent interface: using emacs might be tough on mobile, but there are some pretty good web interfaces for it [2]. I personally use Orgzly [3] and syntching on my Android- Standard protocols: custom scripting does anything. ical is pretty easy to handle, not sure about webdav.- FOSS: check- Multiple calendars: yep, via Org agenda [4]- Subtask support: As deep as you can go- Custom logic: via emacs scripts (or some creativity if you're using other clients)- Markdown notes: yes, minimal differences between org mode and markdown[1] https://orgmode.org/[2] https://github.com/DanielDe/org-web[3] https://www.orgzly.com[4] https://orgmode.org/manual/Agenda-Views.html