Kenya - right now just need the test on arrival - except emirates and etihad require it on departure. To get there, you have to either fly back to JRO and then to NBO, transfer to wilson airstrip and fly to the mara - pretty much all day - or you can get a bush flight that lands on the tz side of the border, minibus over, then another bush flight on the kenya side to the mara - quicker but also a hassle. maybe combine kenya with rwanda on another trip.
Sayari is mainly about the river, but they do have a microbrewery.
Have you looked at Jabali Ridge? looks like your style, you can get a flight from Serengeti to Ruaha. and Ruaha is much different terrain than the Serengeti. .