The simplest solution is the best when it comes to backups (and almost everything). Since you don't need always online, we can eliminate cloud and NAS. Use a 8Tb portable Drives as your backup is enough. With more elimination and compression, you can likely get your storage needs down further. You can use another one for offsite backup. Keep it at a different location like your office or a friends. Use some software like Restic or Borg to encrypt and maintain snapshots of your backup. You can find systemd scripts that automate all of this. Lastly, Periodically test your backup works as expected.

This is what I do basically, but additionally I maintain the current year of photos on Google photos too and use Drive for personal document storage so that its like a live 24x7 cache of recent data. This has worked for ~10 years.