X10 with X6S extender or X10 with Orbi

Using an Orbi RBK50 as an AP is viable, and likely a single satellite will be enough if you are using it in conjunction with the R9000.

The EX8000 has pretty much the same radio topology as the RBK50 (2x2 AC for client facing, and 4x4 AC for backhaul).  That should work too, as you won't get the full benefit of Orbi if you are blending it with the R9000 anyway. 

The EX8000 will let you use the both 5G radios as client facing (because you are using ethernet backhaul).  Orbi isn't doing that (though I am hoping it will at some point).  The Orbi likely would give you smoother roaming between the base and the satellite - but there would be no difference when roaming between the Orbi and the R9000 (compared to roaming between the EX8000 and the R9000).

I'm thinking the EX8000 would work out well.  If you have some flexibility with the router placement, I think you can get by with one extender.  So maybe start with that, and add a second one later if needed.