Deleted my post... sorry.
Haven't looked at the exact set up on your board, but can offer the following advice.
1. RAID can be created on ports 0-5, keep in mind that one or more of these ports will likely become un-useable if you install m.2 SSD. There is nothing bad with using the secondary ASmedia controller in AHCI mode for single disks. Just keep the following in mind. If the controller is only allocated one PCIe lane, then two disks will share this bandwidth, if only one disk is connected, performance is not "split".
2. You can absolutely use the m.2 sata as a boot drive. In general, configuring and installing as UEFI is recommended for best performance and boot times. If you have more specific questions, please reply