Set up fstrim to run once a week or so and scrub your disks periodically. It will not fix any errors unless you have a RAID but it will still tell you if there are any corrupted blocks. If you delete large blocks of data you might want to rebalance every now and then. You really shouldn’t need to do maintenance on a filesystem.

Use something that supports CalDAV and use a sync app (DAVx5 for Android, for example). This will need a computer that hosts the calendar, something globally accessible running radicale or a similar caldav/carddav server. If you can’t host it yourself email services typically also provide CalDAV access. GMail used to provide CalDAV endpoints, not sure if they still do. Fastmail provide CalDAV calendars, but it’s not free.