This is the “allocated” (used) storage, and there’s 35.1 TB still available.
The enclosure obviously has some kind of controller, but I could not find its reference in the specification or data sheet. I would expect USB to be an extra adapter on top of the SATA controller, but maybe there are two different data paths.
Reported progress is just non-linear.
ZFS wants to access all drives in parallel during a scrub or resilver; if the controller is essentially presenting one drive at a time, everything will slow to a crawl.
Honestly, I would power down the server to avoid aggravating the situation, shop around for a SAS enclosure and HBA and move the drives there.