I'm interested in moving off Dropbox in the same way, and B2 sounds interesting. Is there an app comparable to Dropbox working with B2, or did you have to build that app yourself?

I used RClone to sync to B2. They've got an iPhone app that you can download to browse + download from the bucket, else the website itself.

There might be further reasons for it, but I mostly use it as an offsite backup that my wife could upload to -- so usability was a plus -- but not worth that kind of premium.

Not the parent, but you can do this with Nextcloud. It supports using object storage as a backend, and B2 is compatible with S3.