So I rebooted into Big Sur Recovery, cleared the NVRAM, blessed the Big Sur EFI, then rebooted. Big Sur booted as expected, even with the Mojave drive in bay 1. So I shut down the machine, installed the PCIe card with the SSD housing Windows 10 and powered on. It booted directly to Windows. I rebooted holding the escape key, but it didn't bring up the OC menu. It just booted straight to Windows.
I thought I followed your instructions to install Windows 10 perfectly, but it seems I didn't and it's in UEFI mode or something, because every time it boots, it ignores everything else in the machine.