The touch hole liner has a .063 vent hole (not shabby) AND is slightly coned from the back side (also good thing)
This photo shows the vent liner removed and looking down into its threaded hole....here we see what I will call the 2nd powder restriction! I feel the threads should break out into the center chamber but you can see a reduced diameter at the bottom of the tapped hole....that reduced area is only .118 diameter so if the center chamber actually filled up the powder has to turn a 90 degree turn and go thru this small .118 hole to actually fill up to the back of the cone touch hole liner....I bet this almost never happens!
I propose removing this restriction to help the powder "turn the corner" I think this could be a huge improvement on ignition.
The step at the bottom of the threaded vent hole, is good. This provides a "base" for the vent, which reduces actual pressure on the vent itself.
The went us quite small in diameter.
Your drive and willingness to 'modify' the gun to make it better is commendable, however I have to wonder is it worth this "work".
Boring out the flash channel will certainly help with ignition as it will allow poured REAL black powder access to the pan flash. When we load out flinters, then look at the vent
we see a powder granule right there at the vent. This is as it should be. Some people always prick the vent before priming, but I don't see this as a necessity.. When I see powder
right there at the vent, I know ignition will be quick and sure.
Do you now how to knapp a flint to re-sharpen it? If not, you should be able to search knapping a flint right here at ALR and see a demo. I think Taylor did a how-to on this which should be available.