Nice to see the tuning method worked well!
For your flat, since your FOV is filled by the disc, orient your FOV near the area you're about to image, but filled by the disc so that there's no edge in the FOV, then defocus until the features are gone. Fill your histogram to 67~72% or so. Create the flat frame (FireCapture) out of any number of frames (if super noisy, use more frames to stack out noise from your flat). Then re-focus after the flat is applied. Now slew over to the feature you're going to image without moving the camera at all and change exposure to whatever it takes to fill histogram to the same 67~72% (or the range you chose before). The exposures need to be similar so that the flat works.
Lastly, maybe try to use a 2x teleconverter. Your SM60II is generating a F6.6 focal ratio. The HA 656nm wavelength will critically sample on your 224MC's 3.75um pixels at F14. Your 3x teleconverter is significantly over-sampling this wavelength at F20. So drop down to a 2x for F13, which is as close to critical sampling as you can get for this combination at this wavelength. While imaging, capture in 8bit mono/RAW.