For 2.4Ghz WiFi routers, it is true that only channels 1,6,11 are separated enough to not overlap at all.

That said, moving some of the WiFi 2.4Ghz routers to the intervening channels of 3 and 8 and leaving others on 1,6,11 will often significantly reduce the overall interference and improve speeds and WiFi consistency.

In other cases I will move all of them away from channel 6 and spread them across 1,3,5,7,9,11.
I will also ensure that the units closest in channel numbers are physically separated as far as possible.

Any form of channel bonding ('wide' 40Mhz channels) on 2.4Ghz tends to make a hash out of sharing the spectrum with neighboring WiFi. I generally turn it off and do my best to select minimal interference channel allocations.

I understand that WiFi routers are supposed to graciously interleave their signals in time and channel but I find that many routers simply do not do this very well. I suspect that if a router cannot clearly 'hear' another router it will ignore it when choosing its own configuration.

Also, many WiFi routers apparently only scan for channel interference when they boot up. Once operating they seem to not notice the arrival of other routers on the same channels.