Fossify Calendar is a standard android calendar that pulls from the built-in android calendar storage. You can use any standard calendar like Fossify or Etar to mention two open source apps, much in the same way that you sync your nextcloud contacts to the built-in android contact storage. Any standard contacts app can interact with your nextcloud-stored'n'synced contacts.
Running Nextcloud for tasks, calendar and contacts on a self-hosted server is one of the more private solutions.
Finally my moment to shine with incredibly niche knowledge!
Joplin, while it has the ability to encrypt the sync target (even if it's a local folder synced with syncthing) does decrypt the content in the app data folder. The notes are in an unencrypted database while all attachments just hang out in the attachment folder.
This leaves the content vulnerable if the computer is compromised. But then again, apps that keep stuff encrypted at rest still have to decrypt it to memory - leaving the content vulnerable if the computer is compromised. 🤷♂️
All in all, Joplin is definitively one of the great, more secure note taking apps.
I always wondered why there weren't any nuke batteries available. We have had the technology for decades.
Sure, many jobs require people to use unethical, but legal tools and methods. I used to work somewhere like that - and what ever OS they had installed was irrelevant because it was their equipment, their systems and I didn't use work stuff for personal things.
But really, are you describing someone dependent on a job or someone dependent on certain windows-exclusive applications? Are they forced to use their personal equipment for work stuff?
How much privacy respect do you feel you need? Self-hosting that stuff is the holy grail.