Hi Jonathan,
I see you have a backup of your RAID 1 volume.
What you can do is to access the RAID BIOS by pressing Ctrl+I during POST and then reset all disks to non-RAID. After that you can create the RAID 5 volume from the RAID BIOS. As soon as you create the RAID 5 volume all the information that remains in the hard drives will be completely deleted. Once you have the RAID 5 volume created you can install Windows and then Intel RST.
In case RAID 5 option is not available in the RAID BIOS then there is a chance an option called Intel Rapid Recovery is enabled in the BIOS. When that option is enabled a RAID 5 volume cannot be created.