No, routers do not require much CPU power. Typical consumer routers often have really limited CPUs. The biggest issue is getting enough bandwidth out of the Eth

No, routers do not require much CPU power. Typical consumer routers often have really limited CPUs. The biggest issue is getting enough bandwidth out of the Ethernet ports. For example, a Pi 3B+ can only handle 200Mbit or so. Whether that is enough for you will depend upon your WAN speed. Having said that, I'm less sure about your use case. You might could load OpenWRT on it and set it up to relay a wireless signal, but I doubt the performance would be great.