Everything that you can connect with ethernet do so. That will eliminate a lot of throughput issues there and like me, you are lucky enough to have your house wired for ethernet. Even if your media boxes and/or Smart TVs only have 100 Megabit per second NICs in them. Hardwire them. Look for...
I have the RT-AC68P as my main router and the RT-AC56U as my AP. The 68P and 68U are very similar and I would definitely recommend getting the 68 as a replacement w/ Merlin's FW.
In your main router you need to create a static IP for your AP router and the connection between the two should be a LAN port of Main router to a LAN port of AP, not the WAN port. I have RT-AC68P as main router and RT-AC56U as AP connected to each other via ethernet cable and my setup works...
I would change the channels on your network like in the 2.4 ghz mode to 1, 6, and 11 if applicable to prevent radio saturation and use different SSIDs and passwords also for troubleshooting reasons and security issues. Clients these days remember that stuff pretty easily these days.
I had the same problem w/ my PS3 when I had it connected wireless. It would take almost 2 hours to d/l a 2 GB demo w/ a 50 Mb internet connection. I then used ethernet connection and now it takes like 20 minutes to d/l 2 GB demo. Go wired, it is more reliable.
I have the RT AC68P as my main router and the RT AC56U as my wired AP and I have 5 people in my household constantly using our home network or surfing the web w/ no problems. A "1900" megabit router will be plenty of enough for your family as we also stream as you do. Go w/ the RT-AC1900P as...
I always recommend using different SSIDs and different passwords for each SSID. 1) for security 2) troubleshooting...if you have "AFC" and "NFC" SSIDs and your connected to "AFC" and you having problems and then you connect to "NFC" and it's working fine, then you know where the problem lies...