Wall mounting W78H, screw types

[SeanTek2020]

I am trying to mount the base of W78H to the wall. The base is currently mounted, but it is hanging at an angle. I am using two drywall flat countersunk screws inside drywall anchors. The head is flat on top and is tapered with a curve on the bottom (the side that connects to the shaft). I think this is the wrong kind of screw.

For mounting into drywall, what exact screw head is recommended and what exact size of head to work with the W78H? For example, would a truss head screw be a better match to ensure that W78H hangs at a 90 degree angle from the wall? Please provide the best screw/screw head information. Thank you.