I think you might be confusing Backblaze reading files and how Dropbox/OneDrive/Nextcloud/etc. work. NC doesn't enable this by default (I don't think), but Windows calls it virtual file support. There is no avoiding filling the upload buffer, because Backblaze has zero control over how Dropbox downloads files. When Backblaze requests that a file be opened and read, Windows will ask Dropbox or whatever to open the file for it, and to read it. How that is done is up to whatever handles the virtual files. To Backblaze, your Dropbox folder is a normal directory with all that that entails, so Backblaze thinks that it can just zip through the directory and it'll read data from disk, even though that isn't really what's happening. I had to exclude my Nextcloud directory from my Duplicati backups for precisely this reason -- my Nextcloud is hosted on my server, and Duplicati was sending it so many requests it would cause my server to start sending back error 500s.

And no, my server isn't behind cloudflare, primarily because I don't have $200 to throw at them to allow me to proxy arbitrary TCP/UDP ports through their network, and I don't know how to tell CF "Hey, only proxy this traffick but let me handle everything else" (assuming that's even possible given that the usual flow is to put your entire domain behind them).

No, I'm not confusing anything.

Dropbox and onedrive can handle backblaze zipping through and opening many files. The risk is getting too many gigabytes at once, but that shouldn't happen because backblaze should only open enough for immediate upload. If it does happen it's very easily fixed.

If it overloads nextcloud by hitting too many files too fast, that's a legitimate issue but it's not what OP was worried about.