Question still remains: How can one "roll back" to Win 10 1709 on a fresh install, providing that 1803 release is not good for mining?
You cant.
But it looks like you can download both 1709 and 1803 from MicroSoft.
https://www.microsoft.com/en-gb/software-download/windows10ISOIf you have accidentally updated from 1709 to 1803 you can rollback (within 1 month?) by selecting Setup - Recovery - Go back to the previous version of Windows 10