Tutanota’s really the only option except for Proton Calendar, whenever that happens.
Alternatively you can self-host your calendar (many ways to do ics self-hosting) and connect to that with one of the calendar apps (Fantastical, etc), all of which are not privacy-focused and will very probably do telemetry. The web interfaces are not mobile-friendly afaik.
If all of that doesn’t seem worth it, you have to ask yourself the following:
- Is Apple’s stock calendar app bad enough vs. the extra effort/money in placing your trust in a third-party that you can’t verify either? Is what you’d put into it that sensitive? Could you code your entries?
- You could install a tui for a private calendars on some linux/bsd/* box you have and ssh into that
- Just rule out keeping a calendar on iOS for now
- Consider a task management app as an alternative (many of them effectively are calendars, if you set them up that way) … but same caveats about telemetry, unless they expressly state otherwise and have a policy that backs it up.
Or hey, maybe this is your moment, build that thing for other iOS users! Tutanota and Proton are service providers, I doubt their implementations will be relevant to anyone but their services even if/when they get open sourced. (problem: you need $99 and a mac to build that… back to square one.)