I think it’s a matter of physics really and Apple’s recommendation. Each time the Extreme is ‘extended’ I think it’s bandwidth is halved. Also each Express will try to extend the strongest signal in its region, which could be another Express and not the Extreme, if you see what I mean.
Get an app which gives an indication of wireless signal and use it as you move around the house. I use one on my phone called WifiSweetSpot. Works well.
You have to use an Express to extend the Extreme where the signal is strongest, it’s no use trying to extend an Extreme at the end of its range, that’s not how it works.
My aim with my two Extremes (one extending the other) was to flood my house with a decent signal then get the Expresses to ‘join’ to that signal.
My house is a 4 bed semi built in 1956 with a loft conversion. The Virgin hub and first Extreme lie in the small bedroom on the first floor. With just one 5th generation Extreme I can cover this area quite adequately, even into the loft, but I wanted to extend to our garden so I’ve put a second Extreme in the dining/ garden room which covers to the bottom of the garden. The Expresses just join this system. It is rock solid.
I pay for 100Mbps from Virgin so I’m happy with this system. It gives me 100Mbps all over the house (loft about 80Mbps)
If I were you I’d change all the Expresses to ‘join’ and see how it goes. Use the app to see where the signal is poor and switch on each Express In turn to ‘extend’ and see how the signal changes. It really is voodoo and suck it and see.
I hope this makes sense.
By the way, any system is better than the Superhub in router mode.