The volume is 16TB, encrypted, Google Drive (GSuite). The cache disk is a 240GB SSD, FWIW.
So I've had some progres with chkdsk /offlinescanandfix. The only errors it found were free space marked as allocated in both the MFT and the volume bitmap, and then I guess it errored out with "An unspecified error occurred (6e74667363686b2e 1475)"
A cursory search for the error code doesn't turn up anything of substance, but I'll look a bit harder after work.