How should be anything faster as long as your link speed on both repepeaters going directly to modem is double of your future G.fast connection?
And you get this speed with both, 5GHz and even with 2,4GHz (possibly fallback), nothing to optimize …
Before you change anything make speedtests when your line is changed. Everything above 150Mb/s over WLAN is done very well.
And let your devices decide which band to use, if backhaul uses 5GHz its still ok clients using 2,4Ghz as long as they still get almost full line-speed.
You cant change anything on mesh topology as they BOTH are connected DIRECTLY to master router, you can clearly see this on your first picture. To use other way would mean to have the other repeater between actually used one and master router, thats worse!
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