Wood burning in open fireplaces is a lost art. Most custom masonry fireboxes vent into a oversized or poorly designed smoke chamber where the smoke must find its way to the incorrect size flue liner. I’m sorry but the problem is not with any one area but the whole chimney. It may be an outside chimney as well, which cools the chimney and the exhaust passing through it.
That’s not to say there aren’t magnificent fires burning in many open hearths, but as you say not in yours.
In order to truly remedy your problem I would engineer a retrofit system that’s design incorporates modern materials and knowledge about the trajectory, velocity and overall aerodynamics of a column of wood fire smoke.
Then you can overcome environmental factors that adversely affect the process of pyrolysis in your fireplace.
Hope that helps.
Good luck