I would do what seems obvious, ask Getz his recommendations.
Otherwise I have the heretical suggestion that you cast your round balls of modern hard bullet alloy, that is some mix of lead with both tin and antimony. Some years ago one of the Lyman handbooks had good info on such alloys.
Fast twist is nasty with round ball because the soft lead ball strips the rifling, goes right thru without getting much spin applied to it.
Anecdotal evidence: About 1968 a friend, who was an excellent .45acp shot, reloaded a lot. I influenced him to take up muzzle-loading. He bought an Eye-talian percussion pistol, rifled with a large number of fine shallow grooves. Couldn't hit a @!*% thing with it. Thinking like a .45 shooter he then cast his balls of hard modern bullet lead. After that he won all the pistol matches at Old Westmoreland Rifles. I of course do not know, but I think it may have galled the guys with all their fancy custom made guns, flintlocks with backward locks, etc, that they were beat by this cheap pistol. They were, of course, simply beat by a very good pistol shot. Who violated muzzle-loading rules by using hard-cast round balls.
What say you, Mr. Getz?