My devices backup via sync to my NAS and then monthly I backup my NAS to an external hard drive. I would like to backup to Cloud storage but I haven’t found one that meets my needs of 5TB of personal storage. Microsoft OneDrive limits to 1TB per person with their 6TB family plan and Google Drive only goes up to 2TB.
Wow! 5 TB of data is a lot. What percentage of that data is actually changed on a daily or monthly basis.
I have a paid Dropbox Plus account that provides 2TB of storage. However, I also have free accounts with OneDrive, Google Drive, and Box. I have some types of files backed up on each platform. I use Google Drive and Box to store files that rarely change like photos and music, software installation files, etc. Dropbox syncs critical files, many of which change routinely, with all my computers and my NAS. OneDrive stores files that are generated by Office 365.
I am sure that if you look at the files you are storing, you can divide it up among several platforms so you do not need an expensive account.
2nd for Backblaze. You'll want to use the B2 product with the backup software of your choice. The consumer product is client software + storage for PCs/Macs only.
Have you thought about the possibility of having a second nas that is remote and that can be synced with the main one. Once you put something on an external cloud you can't ensure the security of your data. It could be a purchased nas or something simple that just has the storage attached to it. You can do this fairly easy with something like Unraid with Wireguard VPN's between the devices.