Just spitballing here, but if I read this correctly, you pulled the Windows drive, installed Mint, and then put the Windows drive back in alongside the Mint drive? If so, that might be the issue.
UEFI firmware looks for a special EFI partition on the boot drive, and loads the operating system’s own bootloader from there. The Windows drive has one. When you pulled the Windows drive to install Mint on another drive, Mint had to create an EFI partition on its disk to store its bootloader.
Then, when you put the Windows disk back in, there were two EFI partitions. Perhaps the UEFI firmware was looking for the Windows bootloader in the EFI partition on the Mint disk. It would of course not find it there. In my experience, Windows recovery is utterly useless in fixing EFI boot issues.
It’s possible to rebuild the Windows EFI bootloader files manually, but since you don’t mind blowing away both OS installs, I’d say just install Mint on the second drive while both of them are installed in the system, so the installer puts the Mint bootloader on the same EFI partition as the Windows one. With the advent of EFI, Windows will still sometimes blow away a Linux bootloader, but Linux installers are very good at installing alongside Windows. If it does get stuffed up, there’s a utility called Boot-Repair, that you can put on a USB disk, that works a lot better than Windows recovery.