Over the years, I’ve made my best attempt to get my BPE rifles to shoot with black powder.
I’ve actually had good results with Old Eynsford powders - usually 2F and softish grease groove bullets.
I have, however, had very good results in a couple rifles using 4198 and a backer rod filler.
My thought is that if I can replicate the original loading as closely as possible and succeed, then there is no good reason to go to a NfB load.
The nostalgic side of me likes the idea of using these fine rifles just as they were in period to experience and connect with our predecessors who actually used these rifles as tools rather than novelties.

I am intrigued with duplex loads though. Haven’t tried that yet and could be the best option in a hard to regulate rifle.