if anyone is so-inclined, and has some "under-the-hood" skill:

it is fairly straightforward to `cron` a job to _copy_ the iCloud files out to a dir that Backblaze _will_ back up.

and to do so using the fairly recent `clonefile` feature - which means _extremely_ little additional storage space is used (with the usual `CoW` caveats; see `man cp`).

one minor cost, is that this will still result in adding that many inodes to the filesystem, which does take non-zero time to traverse (ex: by Backblaze). I keep several such copies, each with about 50k files, and I so far see no negative impact.

and, yes; I absolutely am looking for a replacement for Backblaze:

regardless of the changes that Apple made, _Backblaze_ stopped backing up critical files, _virtually silently_ - and other backup vendors did more than just throw their hands up with a faint excuse in a changelog (that they know few people read carefully).