I rebooted many times with known-good USB drives in different ports. When I get to the boot choice (hit ESC at BIOS splash screen, follow prompt OR just F9 at BIOS splash screen) I see the internal hard drive, a blank but selectable line, then CD/DVD drive. I've gone into the BIOS and there's no secure boot option or anything but I can get into the boot order and the USB drive shows up there.
Select the blank line and hit Enter. This didn't work every time and I don't know why, but try it again. The time it did work I was saving time by loading up the USB ports with known-good USB drives. So then there were two blank entries. I tried the first one and now it's installing Win10