Also @timmmay can I install the Backblaze personal software on a server OS and backup to their cloud (e.g. on my home server) - or does server OS always require the business plan and additional B2 storage?
I don't know about their server stuff sorry.
They don't have a cloud. They're a software provider that lets you back up to the backup storage of your choice. BackBlaze B2, AWS, Azure, Google, Dropbox, any many many more.
So you mean the personal Backblaze you have to pay storage separately, like Cloudberry? That's not what I read on their Backblaze consumer page.