Since you'll still have PCIe slots open after installing the RX 570 and RAID card, you could install your macOS SSD to one of these OWC PCIe 2.5" drive cards instead of the optical drive bay. Leave your Windows drive connected to a builtin SATA port since it's tricky to run Windows off drives installed in a PCIe slot. You'll also have the added benefit of faster SATA III speeds for your macOS drive.

Now that you have a free connector in the optical drive bay, just buy one of the SATA to dual Molex power adapters you mentioned above. Since the connectors in the optical drive bay are combined SATA power and data, you'll have to modify the SATA power plug.

Edit: Also, to be safe, make sure you are running a Legacy install of Windows 10 instead of a UEFI install. UEFI Windows 10 writes Secure Boot certificates to the firmware of the Mac Pro and can eventually brick it. However, it is possible to get around this by installing the OpenCore bootloader to the EFI partition of your macOS drive. OpenCore will give you a boot screen and drive picker with your RX 570 and protect your Mac Pro from UEFI boot certificates if you choose to use a UEFI install of Windows (Just make sure your Mac Pro never tries to boot the UEFI install without OpenCore). You can also install the Windows SSD on a OWC PCIe card since the issue I mentioned in the first paragraph is due the Mac Pro being unable to boot legacy windows from a PCIe installed drive, only UEFI Windows. Check out this forum thread for all information pertaining to OpenCore on the Mac Pro. You can even run Catalina natively using OpenCore, but I choose to just run Mojave on my system. I highly recommend reading through most of the thread.