I had the same problem while I was trying to extend a heavy haul road for my woodlot. I could spread the gravel out reasonably well, but the finish grade was nearly impossible to get with a 3 point hitch mounted grader blade. The problem with those is, as the tractor rides into a hole, the grader blade dips down, and coming out of it; it goes up. For a smooth, flat road along its length, that is no good.
To cure this, I welded my grader blade to a goose-neck shaped I beam that bolts between the front hitch of my log trailer, and the back portion of it. This lengthened my trailer to 18 feet giving me a much more smoother road. The walking beam suspension of the trailer also allows the blade to stay flat even when one tire hits a rock or bump helping to make a smoother road as well, And of course the goose-neck design allows the grader blade to be angled in each direction.
It does lack a tilt function, but honestly I can reestablish any crown with my bulldozer much easier, and then just smooth up my roads with my grader. The worst issue is that it can be a pain to bolt and unbolt from the trailer. But as for a finish, it leaves my roads satin-smooth.
If you really want a smooth finish, that is what it is going to take, because you really need length to span the most dips and hummocks. The distance between the rear wheel of the tractor, and the first wheel (or only wheel) of the trailer will make the blade rise and fall half the distance. In practice what I have found is, the first drag will mean deep cuts and then places where it just skips over, but on the second pass i set the blade to just kiss the ground and it smooths my road perfectly...two passes, that is it!
Do you have an old trailer frame you could mount your grader blade on?