I do not have a solution to get your Nextcloud instance quickly up and running again, if you have a hardware failure, but I can share my backup strategy.
I create a daily dump of my MariaDB database into a file. Then I use a backup tool to create a backup of this file, my Nextcloud folder, and the data folder onto a different hard drive. I use Borg backup, which I can highly recommend. It supports encryption and connections over SSH, which can help you concerning offsite storage and if your second hard drive is located on a different machine. But any other good backup tool, like the before mentioned Duplicity, will work. After the backup is completed, I use rsync to copy everything to a friend’s house. He does the same thing with his data onto my home server. We each have implemented a SFTP only user over SSH with key authentication. With the encrypted backup and SFTP only, nobody can read your data and you do not have access to the server, except for the files you store there.
If you can offer to store your friend’s data on your computer too and not asking for full access to their system, that might make finding an offsite storage location easier. Maybe you can even get an additional hard drive and a SoC device like a Raspberry Pi and place this at your family’s or friend’s house. This would be a cost effective option in my opinion.