You cannot use the Intel RST support for RAID arrays across NVMe drives without also using UEFI. If you install the O/S for Legacy, it's true, you will not be able to boot from the array.
I use Rufus to prepare my boot media using GPT support for UEFI. I have had no luck at all with Windows 7 and UEFI without using this tool. The Microsoft-provided tools typically do not work.
Hope this helps,