Your M.2 drive has a UEFI install. You can tell by the EFI system partition.
If you want to do another install of Win 10, boot the install media to the UEFI and install. The boot files will be put in the other EFI partition and the OS will go on the drive you designate.
Do you ever boot into your other install on the M.2 drive?
"boot the install media to the UEFI and install" - I'm not clear on what that entails.
"Do you ever boot into your other install on the M.2 drive?" Also not clear what that means, other install. Sorry.