Hi Mim
As you have a thickened endometrium while being on the reduced progesterone dose ie less than the licensed amount for the cyclical regime - of 200 mg x 12 days per 28 day cycle - then it would seem sensible to go back to 200 mg per cycle to thin the lining down again ( provided no abnormailities have been found after hysterosocopy etc).
The presence of a bleed does not automatically mean that the progesterone dose is sufficient to keep the endometrial thickness within normal limits, ie that all of it is being shed. My endometrium was thickened on last scan (though I am on longer cycle) whereas previously it never was, when done immediately post-bleed.
I don't think increasing your cycle to 12 days would make that much diference in terms of thinning the lining though it may work as a maintenance dose once your lining has thinned to a normal level?
If you can cope with the 200 mg dose for a few months that would be my choice, since like you I don't want to use synthetic progestogens.
Hurdity x