Stop buying routers. Instead get a Raspi and USB wifi adapter capable of master mode.
The Pi has a single 100Mbit Ethernet hooked off the USB 2.0 bus. You're putting both the Ethernet and the wifi on the USB, which is going to get congested.
A typical home router has one or two gigabit Ethernet ports hooked directly to the SoC, with one of the interfaces connected to an internal manageable switch. It has one or two WiFi interfaces connected to a high-speed, low-latency bus. The WNDR3700 is a good example of the type of hardware people like to run OpenWRT on.
As far as I am aware, there is no cheap, hackable board that has the kind of connectivity you need for decent WiFi router.