I'm new to RBE970/971 but old to all this netz stuff

Thanks... you had me chuckling as I imagine our having very similar experiences! Interestingly, Amazon had a 'deal' on the Orbi 97X's and limited you from buying more than one 2-RBS pack in a single purchase. So, when I first received the 3-pack and one 2-pack, I set up the RBR and 4 RBS's. Very stable results for the first 3 days. All wired client devices were correctly seen by the Orbi app. It was when I added the 5th RBS that all hell broke loose. Things that were stable before, were no longer stable. reboots, loss of RBS connectivity (per the web and app pages) were declared. From a basic network capability however, these 'not connected per the UI satellites were in fact on the wired network. I could ping them. Moreover, I took my 200 ft CAT6A extension cable roll and attached one of the RBS to it. I then ensured my iPhone was connected to 'that' RBS and ran near gigabit WiFi over the so-called "not connected" satellite (again according to the router, the Orbi app and web page). 

My focus was solely seeking stability in the Orbi RBS/RBR wired network without regard and consideration of any client device attachment or other wireless matters. When NG support told me that 9.13..;. would not help stability and that I should reduce to four satellites, I realized I was in serious trouble. 

Your point of balancing distance when using wireless backhaul is similar to what I've seen and done; it's actually the reason I do not mess around with wireless backhaul as a general practice. N.B. I do use MOCA 2.1 adapters in installs where it is impossible to run new Cat 6 or better cable. Though I've not tested it at this speed, they're said to support 2.5 gbps. My test gear allows me to test & validate/qualify up to 1 gbps.

I use an app written for windows platforms to assess the wireless neighborhood. Company is called Metageek that make a USB dongle and software called Channelizer. re: metageek.com  With this product you can find identify the types of sources occupying 2.4, 5 and 6 ghz spectrum. It data logs the RF energy so you can go back and analyze what is going on. With a directional antenna, you can actually 'hunt down' the sources that may be interfering with you. They provide characteristic profile RF energy curves that help you determine whether the sources are Bluetooth, zigbee, zwave,  nanny cam, pro-security cam, or other devices. My approach anecdotally speaking is to get a satellite to within 25 feet of the client's most frequented locations. I've found that the wireless throughput seriously drops off when distances go over 25 feet (some studies show that WiFi throughput seriously drops off at that distance); that's without any walls in the path. I end up adjusting transmit power get optimize performance and minimize interference; with all satellites using wired backhaul, the problem reduces to only being a client device problem and not a satellite to router wireless problem.     

Wow, 1862 construction! I'm sure you've had your WiFi challenges with the different materials used in your home over the century. Impressive. 

Were the drop off issues meaning a drop-off in throughput as WiFi distances increase or the client device randomly disconnects from the Wi-Fi. If it's the later, I was wondering whether you suspect MLO operation? My past experiences with 'bonding' or 'stripping' methods over parallel paths have been very erratic. It would be interesting to know if turning off MLO (or any other 'smart connect' function would improve the drop-off situation.   For decades we've had separate SSID's for each band (2.4, 5) can device drop-outs hadn't occurred. it wasn't until some bonding technique to leverage 2.4 & 5Ghz and now 2.4, 5 and 6ghz did device drop offs started happening for me. Two other related questions are (1) whether the drop offs are occurring based on 'age' of device (meaning older verses newer devices), and (2)  whether the drop offs are due by some 'power saving' mode in the device. 

Be well.. thanks for your interesting dialog...m