I think the idea of magic properties with regard to fat needs to be forgotten.
It seems extremely unlikely to me that any Class I androgen (one working principally or entirely via the androgen receptor), e.g. oxandrolone is going to accomplish anything that a comparable dose of testosterone will not, given equal estrogen levels.
I think that what has happened is people have misattributed the additional effects (beyond androgenic effect) of correlated lowered estrogen level while taking oxandrolone alone, to the oxandrolone itself.
I don't know of anyone finding that adding oxandrolone to a strong testosterone stack with no aromatase inhibitor added any magic fat loss properties. In that case estrogen stays the same.
I suggest using just the testosterone and enough AI to keep estrogen low normal. Kick up the testosterone by perhaps twice as many mg per week as you were thinking of taking as oxandrolone, so as to probably not be reducing androgen effect. With your above figures, that would be say 600 mg cypionate per week, preferably as 2 or 3 injections per week.
Drop the change-the-dosage-all-over-the-place approach and just do the same throughout. If (if) a stack is what it should be for you in a given week, then something quite different is NOT going to be right for you other weeks, is it.
Unless there is some specific reason having to do with drastic dieting or training change. But even in this case this tapering off stuff is not the way to go. Just stop the injections at the beginning of week 8.