@Gigabit
My answer was maybe too short.
I have set up more then 10 bigger wifi networks based on ath9k+wds. They dont have the lag you was talking about. Thats why i recommend to setup WDS+ath9k. In your example you could try out to use a second WDR3600 and replace the Archer C7 with the WDR3600 to match out.
@jeff
If a "downloadable firmware" is free enough for you, then just stay at the vendor blob instead of using free and opensource software like OpenWrt. The vendor provide in nearly all cases a "downloadable firmware" on his website for the wifi-router he is building/selling.