My answer to maintaining a minimal set of photos on a local machine isn't great unfortunately. The desktop holding all the photos acts like a file server and I connect my laptop to that when browsing images.

There's a way to do it that I find somewhat risky: pay a bit extra for permanent storage. Normally Backblaze will remove deleted files in 30 days (I pay for a year). If you do permanent then you can delete your originals from your local machine and Backblaze will keep them presumably forever. That approach makes me nervous personally :)