Ok, made a bit of progress.
When it boots into twrp, I can run adb reboot bootloader and it gives me the bootloader menu. From there I've tried flashing a new twrp blob but the tablet freezes during the flash.
I can also boot into Android from that menu. What can I do to fix this now?
....aaaaand of course I find Buster99's softbrick fix instructions and all of a sudden fastboot isn't working anymore
Just says waiting for device.
My computer begs to differ:
Product ID: 0x4daf
Vendor ID: 0x0b05 (ASUSTek Computer Inc.)
Serial Number: 015d1884b763f80a
Speed: Up to 480 Mb/sec
Manufacturer: ASUSTeK COMPUTER INC.
Location ID: 0xfd130000 / 6
Current Available (mA): 500
Current Required (mA): 500