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Calendar and tasks sync between devices
(lemmy.ml)
F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
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If you're willing to self host Nextcloud then that could be a solution.
Currently I use it to sync my calendar on my phone and an instance of MagicMirror2.
You don't necessarily need to self-host Nextcloud for yourself, you can pay a provider to do it for you, of course, at a price, or free but with limitations, like The Good Cloud.
You can also try EteSync.
And in F-Droid there are many apps to integrate your Nextcloud instance with your device, my favorite are Ente (for calendar), Tasks(dot)org and Joplin