The solution is don't do VoIP on WiFi. You mentioned the 3 ethernet VoIP phones, do those have issues? Also, unless you're running third party firmware on your Asus router, you're going to start dropping VoIP packets whenever anything maxes out your connection. The packet loss actually isn't really a problem, the problem is that when you max out your connection, whatever cable modem you're using is going to start filling up a massively oversized buffer instead of dropping packets and any new packets go to the back of that buffer and take hundreds of milliseconds before the packet even leaves your building.
The fix if you're stuck dealing with crappy equipment that has massively oversized buffers is to throttle the connection in another device downstream of the modem to slightly under what the modem will throttle you to and then in addition to that you can also implement QoS so that your VoIP traffic is put into a different queue and give that queue a higher priority.
This is all predicated on the assumption that your ISP is actually providing what they say they are 100% of the time. If you pay for 5 Mbps upstream, set the QoS to 4.5 Mbps and your ISP delivers 4 Mbps 10% of the time then for that 10% of the time your equipment will be sending too fast and you'll go back to filling up that oversized buffer just like before.
The fix if you're stuck dealing with crappy equipment that has massively oversized buffers is to throttle the connection in another device downstream of the modem to slightly under what the modem will throttle you to and then in addition to that you can also implement QoS so that your VoIP traffic is put into a different queue and give that queue a higher priority.
This is all predicated on the assumption that your ISP is actually providing what they say they are 100% of the time. If you pay for 5 Mbps upstream, set the QoS to 4.5 Mbps and your ISP delivers 4 Mbps 10% of the time then for that 10% of the time your equipment will be sending too fast and you'll go back to filling up that oversized buffer just like before.