What do you mean by USB hard drive enclosures? Are you limiting the RAID (8 bay) throughput by a single USB line?! That's like towing a ferrari with a bicycle.
I have one enclosure plugged into a USB 3.0 line, another plugged into a "super speed" line, and one plugged into a Thunderbolt line (shared with my 10GbE Thunderbolt card with a 2x20 gigabit splitter).
This was deliberate, each is actually on a separate USB controller. Assuming I'm bottlenecked by the slowest, I'm limited to 5 gigabits per RAID, but for spinners that's really not that bad.
ETA: It's just a soft RAID with ZFS, I set up each 8-bay enclosure with its own RAIDZ2, and then glued all three together into one big pool mounted at `/tank`. I had to do a bit of systemd chicanery with NixOS to make sure it mounted after the USB stuff started, but that was like ten lines of config I do exactly once, so it wasn't a big deal.
But often after extensive use, `dmesg` would complain about problems when talking to the drives, e.g. drive not responding or other strange error messages (forgot the exact text but very irritating and google-fu didn't help). There were also problems with SMART commands passthrough and drive power management e.g. sleep/standby adjustment which wasn't reliable when talking via bridge chips.
I use only disks directly connected to SATA controllers afterwards and no such issues ever happened again.
Though even flaky adapters just tend to lock up, I think.
ETA: It would certainly take longer with more data. I've not gotten anywhere close to the 288TB