I would recommend route 3 if possible, it's the nicest of them in my opinion and can be done on a big bike (though a small/medium would be better). Unfortunately with your lack of available days it's going to require slabbing somewhere. You'd be better off getting extra days and doing a 3/2/5/4 circle.
Maybe park a vehicle at the end of your planned route that can carry the bikes back?
The Sisters area if a great place to stage, I live just off route 4 near Sisters and the amount of trails/fire roads in the 3 Sisters area is huge. You could easily do some planning and spend half your time heading south on route 3, then turning around and finding some alternative ways back and be on fun stuff the entire time.
The area south of Christmas valley IS pretty rocky/sandy, but completely rideable on a big bike.
I'll add the last time I rode route 3 was in July, and two spots south of sisters were closed due to snow. Though, unless we get a bunch more late winter/spring snow that won't be an issue this year.
The section of route 4 from Sisters to Prineville is nothing more than a connection route. Just go directly from sisters through Redmond or Terrebonne and catch 4 just north of Prineville. Very nice riding from there east.
2010 690 EnduroR
2013 500 Exc