Good call on the -75, I didn't look too far into it but I will now. Easy enough to switch, so even though I'm looking good now (I'm that guy using ch 4 lol) I'll see how close to 'perfect' i can get
Every trainer for every brand says to use channels 1, 6 and 11 because there will be no overlap and since the neighboring WiFi won't be as strong as yours, theirs shouldn't bother your performance. If your APs are basically in a line from the gateway to the end and you can't see the gateway (In Inssider or other WiFi finder) when you're near the farthest AP, use the same channel as the gateway. You can adjust the power in each to allow this, too. If your computers and other devices that use WiFi have never been connected to the neighbor's network, they won't try to connect, especially if you turn off features like Auto-Connect (in some Apple hand-helds).
Best practice is to hard-wire stationary devices and if you have one or more groups in different locations (like in an AV cabinet),use a switch for those pieces. If you have a spare port, you can connect an AP but nobody recommends using bridge mode to connect one repeater to another repeater. If that even works, the speed and throughput would be abysmal.
For testing purposes, it can be helpful to add a number to the SSID for each transmitting device. When you see that the signal strength is consistent for all or you can't see the gateway at the farthest location and your speed/throughput is good at all locations, remove the numbers so you can roam as well as possible.
Remember- Apple devices leave claw marks when they let go of a connection.