Sunfish,
"Going back to spherical with grinding and then polishing sounds like a good solution." I'd second that, from my reading you need to be spherical and polished out before parabolizing the mirror.
I am still at a very simple mirror for learning, the F4 will be a beast.
Try Mel Bartels site for how to make different pitch laps for the parabolizing of fast mirrors.
https://www.bbastrodesigns.com/JoyOf...abolizing.html
https://www.bbastrodesigns.com/PitchLapCalc.html
PS. I got a flash polish on my mirror enough to test. Revealed some nasty flaws, dropped back to 400 and fine tuned the F/l, then life got in the way for the last few weeks... But no matter what you do, you can always remake the surface of the mirror and go again.