My setup is comprised of 4 Deco M9 Plus units (2 of v1.2 and 2 of v2). They all want to use the base station as the signal source, and that signal is quite weak for at least one of my Decos, so I decided I would run an Ethernet backhaul to another unit on the other side of the house. Unfortunately even with the Ethernet backhaul the Network Optimization leaves all units pointing to the base station as the signal source. I can prove by locking my phone to either the base station or the backhauled Deco, and placing it by the weak Deco  that it would be 5-10X faster if the weak Deco used the backhauled unit as its signal source:

Phone sitting by weak Deco, linked to Base Station = 5 Mbps (which is also what I get if the phone is linked to weak Deco)

Phone sitting by weak Deco, linked to Backhauled Deco = 70 Mbps (close to my max expected speed of 75 Mbps)

And to test the Ethernet backhaul itself: Phone sitting by Backhauled Deco, linked to Backhauled Deco = 70Mbps

So I can see:

i) the Ethernet backhaul is good, and

ii) Weak Deco would be much faster if it was linked to the Backhauled Deco.

Not sure what I can do here to make the weak Deco link the the Backhauled Deco. Does the mix of version numbers matter? Currently my Base Station is v1.2 but the Backhauled unit is v2 as is the weak Deco. Is that a problem? It is quite frustrating to see the units failing at this basic functionality expected of a Mesh setup!