This one sure has gone lots of ways. Key will be if either of those 2 already have 2 ethernet ports. If not you are going to have to add a ethernet board. Without digging around in the specs on cpu I don't know but in general a router function does better with faster clock rate cpu, it does not use multiple cpu well.
I have gotten lost in the thread. I though you said it worked ok if you plugged into the main router. The raw number of sessions seldom is a issue. If someone is running bit torrent it might have issue but even then this is more a memory issue. Even very inexpensive routers can pass 1gbit of traffic wan-lan because they have a special software NAT offload feature that bypasses the cpu.
This will not make your wifi problems better. You still need to keep your current routers for the Wifi radios. The number of wifi sessions a device can handle is done in the wifi chips themselves the cpu is not involved much. Using a wifi add in board in a PC works very poorly, I think mostly because of the location of the antenna. When you use a PC as a router you tend to always use some form of AP for your wifi.
Maybe I got lost here most times you only need to use a pc as a router when you need high speed vpn or your are in some way over utilizing a very large internet connection and are trying to use QoS to solve it rather than try to reduce the load or buy a bigger internet connection.
You should read up on Buffer Bloat(https://www.stoplagging.com/ ), not everyone can afford or have the ability to even get gigabit internet. I have gigabit internet and still had problems with bufferbloat because my upload is maxed out at 25mbps. If someone is syncing or uploading to the cloud, then other people on the network gaming will get packet loss without a good QOS system. Read up on the development of FQ_Codel and the eventual successor CAKE. I installed OpenWRT on my old router and enabled FQ_Codel, it was simply awesome. The problem was the ARM chip inside the router would max out my download speed at about 300mbps using the algorithm. It wasn't powerful enough. Then I looked at x86 off the shelf routers to handle it, and they were $600+. So I built my own router for much cheaper. I bought the processor, motherboard and ram for about $150. The power supply, USB stick and case I had already laying around. The 2 intel retired server cards were $20 shipped for the pair.
I would still use the Intel based network cards I suggested. It's much better for router and server duty. Significantly more reliable than the chips used on most cheaper motherboards.
The method of operation is a little more advanced, but this is basically how corporate businesses do their internet. They have a system rack with a router server, then have wifi access points to serve the areas of the office with wifi.
I then repurposed my expensive Asus routers to be a mesh access point network with wired backhaul. My network has never been this reliable. I used to get dropped packets and occasional internet lockups all the time. Rainbow Six Siege tells you when it's client side or server side on the screen as it's happening. I haven't had any client side lag since my new setup, it just works so well.
The Quad Q6600 is too much of a power hog. The core2 Duo E7500 should be sufficient for your internet speed, might struggle if you ever upgrade to gigabit internet, but should be plenty powerful enough for what you have now. Eventually when you retire your core i7, you can repurpose that into your new router. It should be able to handle well beyond gigabit FQ_Codel traffic shaping.I've got a couple other older computers lying around, would they be better suited to the task?
CPU Intel Core2 Quad CPU Q6600 2.40GHz
CPU Intel Core2 Duo CPU E7500 @ 2.93GHz
Just wondering if one of those might be a better solution for what I'm asking it to do.