I store them on my NAS, and my NAS has a synchronisation with Amazon Glacier which is a very cheap storage service that is designed for cold backups, meaning if

I store them on my NAS, and my NAS has a synchronisation with Amazon Glacier which is a very cheap storage service that is designed for cold backups, meaning if you want to request a backup it can take 24 hours for Amazon to provide it, unlike Amazon S3 where you can access the files instantly but is 10 times more expensive. But you should only need to request a backup if your house and your NAS get robbed which hopefully happens very occasionally in your lifetime.