I ended up with just one Minio docker and then just create multiple containers inside.
The clients are using Duplicati installed as a service, backing up into each their Minio containers. Duplicati encrypts the backups, and they all use different keys.
After backup duplicati sends a http string to https://www.duplicati-monitoring.com, which then again sends me daily emails with status.
I have run a few restore tests, and it all seems to work fine.
All the notes about installation, keys, restore process, etc, is kept at Lastpass. So when things turn bad, i don't need to rethink what to do, just logon to Lastpass and follow my own instructions :)
For me this is exactly what I need, and I consider the problem solved... for now at least.