re: Ten Commandments

I think if you over fill a bit and then block sand with a flat sanding block will avoid the concavity!
My favorite way to fill like that is to finish sand first, burn the pattern and the coat the wood with sealer so nothing wicks into the wood around the subject. Then I over-fill it with inlay material and  drip on CA glue and acetone to cure.
I then sand down to the sealer and take off very little wood.