How to optimize my Orbi Network/Am I getting the best performance?

This seems like a LOT of WiFi radios in a small space. The general practice is to space Orbi units "about 30 ft." apart, depending on building materials.  It appears that several of these are not much over 10' apart.

The house has brick exterior walls. What is the interior construction? (walls, floor)

It is unusual to connect another Orbi router "behind" an Orbi router, as this creates two separate WiFi networks. Even if they have identical SSID/password, devices will not "roam" between the two.

Diagnosing things generally requires software that can report the signal strength at various places.  Often, WiFi device network management will report RSSI and "Link Rate" in numeric terms (rather than those stupid pictures and words like "Excellent").

There is software for Apple products that allows the user to walk around and see how strong the signal is in various places. (not an Apple person, can't remember the name).  On Android, I use "WiFi Analyzer by Kevin Yuan.  There are free programs for Windows that do the same thing.

The reason I bought the Orbi four years ago was I picked up a free "Heat Map" program and plotted signal strength in the house.  LIke your situation, I had great signal where the ISP modem entered the house, but it fell off tremendously at the far end.

A lot of discussion, but "not much help" I'm afraid.