I want a dedicated rear endcap-mount with M8X.75 threads for a pistol instead of the 1/2-28 threads the factory endcaps have.

I don't like all the multiple joints you get when using an adapter.

Might have a local machinist make a custom one. I can measure that the silencer tube is 25mm interior, 27/28-ish exterior, but dunno what the female tube thread pitch or the male thread pitch is on the normal rear endcap - anyone know those? Or know anyone that already has done this and sells such a thing?

Any machinist is going to want the parts in-hand, to do the job. Make sure you stay with the silencer, if you go this route.