Hosting on own device requires decent power supply, including a UPS, and a backup system.
NextCloud backup (link) instructions. Restoring backup (link).
Standard UPS is fantastic but also might be a bit of an overkill for a low-power device like RPi. It seems that a standard USB power bank would do the job just as well.
USB flash stick can simply be pulled out and read on another machine. However, since NextCloud requires user permissions, the USB stick is formatted with ext4. Additional software is required, see ext2read.
On RPi, we can back up the NextCloud database file directly, assuming it is in sqlite3 format.
Links
Contacts can be backed up in several ways.
Calendar can be backed up in .ics format.