I would suggest not so much lathe tool over hang when turning the disk. I hate when there is tool chatter like that.

When drilling and tapping the holes, start the center drills as shown. But when drilling through, CLAMP the piece securely to drill the hole. Then without moving the work piece, replace the drill with the tap, Start the tap while holding it in the drill chuck. You can use the feed lever for a little down pressure, and turn the chuck by hand. Or if you have a strap wrench, you can use that on the chuck. This will assure the tap is perfectly aligned with the axis of the hole.

The recesses turned on the one end of the posts should serve to assure the posts stand perpendicular to the plate. They prevent the post from contacting any distortion that might be around the threaded holes in the plate. The recesses should be concentric with the bolt holes to assure even pressure around the perimeter of the post.

I see no reason to use socket head cap screws. The top of the posts do not contact anything in use.

That is all I see from a quick glance.