It's almost impossible to avoid the overcrowded wifi channels

Speaking as a wireless network admin, the short course on 2.4:

  • In most of the world, there are only 3 2.4 GHz channels you should ever use: 1, 6, 11
  • All the rest of them overlap - the idea that adjacent channels should not interfere with each other is broken in the 2.4 spectrum - accept it and move on. There are only 3, and they are called 1, 6 and 11.
  • Unless you are in the boonies (WiFi analyzer shows nothing if you turn off your own wireless) 2.4 should only be run at 20 MHz channel width, never at 40 MHz. At 40 MHz, there are no non-overlapping channels, since it takes two, and there are only 3 to start with, so as soon as you have 2 APs trying to run 40 MHz, you have interference.
  • Everything that can be wired should be wired. Access points should be as close as possible to the end use devices, and should generally be turned down somewhat from default (almost always maximum) power levels; end use devices generally cannot talk back to the AP at high power level, so a lower power from the AP balances the communication better.

In any high-density environment, 5GHz offers great benefit, precisely because it's terrible at penetrating walls, making your neighbor's AP less of an issue for you - also, in most parts of the world, there are MANY more channels available in the 5 GHz band. However, the terrible at penetrating walls part applies in your house as well, so you may need to wire more access points for good 5 GHz wireless coverage.

And before you go there: wireless repeating is terrible for wifi performance. Good wifi is built on wires and fibers.