RE: This is gonna be tough...
I actually built and flew a very similar design not that long ago. Was more of an eagle, but it handled just like a buzzard. Slow wheeling type flight.....yours may be a bit faster with less of a WS than mine. Flying wings need a slightly more aft CG...usually. I'd err in the aft direction and then move stuff forward as testing provides you with some data.
ZZ.
Was the loudest damn buzzard you ever saw though....lol! none of the others liked him very much.