My email calendars I leave alone, but I use caldav for personal calendar and tasks.
I use radicale as caldav server, and tasks.org on android and thunderbird on computer. Tasks.org works very well
I also use silverbullet (silverbullet.md) for more complex todo lists
I have used them since januar 2019, and I don’t have any complaints. I have only needed to restore backups once - it worked as well as could be expected.
Any issues with backups have always been on my side