Using Orbi RBS750 satellite with RBR850 router

I share your frustration.  If it helps, I found no practical difference between the RBS 750 and RBS 850 in my situation.

I started with an Orbi RBK 853 which consists of 1 RBR 850 router and 2 RBS 850 satellites.  I added an RBK 753S which consists of 1 RBR 750 router and 2 RBS 750 satellites, using the satellites to extend the RBR 850.  All backhaul is wired.  I worked well.  Per FURRYe38 earlier in this thread, "If you're using LAN wired backhaul then the backhaul will be 1000Mpbs for all RBS. Front haul won't matter".  I think s/he is correct.  Incidentally, I bought the RBK 753S at Costco for less than the cost of two RBS 750 satellites.

But, I wanted all 850's so I bought another RBK 853 and used the two RBS 850 satellites to replace the two RBS 750 satellites from the RBK 753S.  (The second RBR 850 is unused.) From my meaurements using Speedtest in my environment, I saw no difference.  However, I did note the dent in my wallet.

That's a long way of saying, perhaps you can consider the RBS 750 vs. the RBS 850 to extend your system.  It worked for me.  And, you'll save $.

I'm keeping the 850 system just because I want it.  I recently upgraded my Ziply Fiber to gig speed and, with the RBR 850, (plus a new ONT from Ziply and some tweaks in the Ziply system), I am consistently measuring 910Mbps - 940 Mbps up and down at the Orbi router.  That's good enough for me.

I am returning the RBK 753S.

Hope this helps.