I am wondering what services do privacy-concerned users choose for convenient cross-device tracking of "things to do" and calendar events.
My main systems are macOS and GrapheneOS, an I've been using a self-hosted solution, running "radicale" app on my own (not rented) private server. This was a simple CalDav server, and macOS's native apps (Calendar and Reminders) worked very well with it. On GrapheneOS I used opensource Tasks app. They all have very good notifications/alerts, can work fully offline if there's no internet, as they don't use stuff like Push notifications and sync only when they have an opportunity.
However now I don't have a possibility to run such server, and I need to move to some cloud-based solution, which raises concerns regarding privacy.
From what I understand, some people use Proton Calendar, that promises to do end-to-end encryption, but they do not support caldav protocol (meaning, we cannot use it with native OS apps), and, what's worse, their apps are not open source, so these apps cannot be audited.
iCloud (afaik, end to end encryption can be enabled for it as well) works only for Mac but not for Android/GrapheneOS.
What other private solutions are there?