What helped me track it down was removing the camera from my efw and attaching my flat panel to the end of the scope at a medium brightness, that made it pretty obvious when I looked down the draw tube.
Fascinating. I, too, have been fighting internal reflections that produce the same pattern - but only with the reducer. I actually removed the rotator because it very easily induces tilt.
I have painted what I could access of the reducer without risking painting the glass and flocked much of the dew shield, and re-purposed an astrozap flexible dew shield from another scope to try as an extender. And I even bought an APM Riccardi 0.75x.
The reflections remain, but it's been a lot easier to give up using the reducer than having to give up on the whole scope. Probably 95% of my targets I would not use the reducer anyway.
I have thought about trying to flock or paint the focuser drawtube, but I always stop when I look at the 20-ish baffles. Did you go ahead with the painting? How did that go?
I thought it was my rotator (Pegasus Falcon) or one of the adapters so I painted those with muso black
I’m currently using the 1x TSFlat 2.5 flattener from teleskop express, I’m clouded out for the time being but I’ll throw the .8x in when the clouds break (at least a week as far as I can tell)
