I think you are doing very good with these. On the replication matter, have you thought about casting the head? This way you can make a dozen heads and of course change beards and even mouth expressions on each individual one but be sure that the helms are all exactly the same. I do understand that you want to hone your skills so that you can replicate it and practice will do that, but just a suggestions (often on pro sculpts I will see heads that look like they were cast in plastic or resin on fresh Green bodies with fresh green hair (even on GW sculpts when a rare Green version is shown).