What’s the best way to distribute the bandwidth & client connections (in my scenario) on our network?

Our setup:

there is a coaxial to a basic Arris modem. That Arris modem is connected to a board of coaxial that then have a main line that feed into the wall where spectrum comes from.

As well, there is a 2nd coaxial that comes from the board of coaxial that goes to a Arris Surfboard. It’s like data modem to allow phones and enternet to work once connected via wan Ethernet. So that Arris Surfboard has a single Ethernet wan port that has a Ethernet that plugs into the Nighthawk wan router. Which allows the nighthawk to allow WiFi and enternet to work. Then the nighthawk router has a Ethernet lan cable that runs from it to the Cisco switch and the Cisco switch then makes all the wired computers and phones working.

What I want to do:

is to somehow make that nighthawk router the one for the Cisco switch so that only the wired phones and computers use it. The nighthawk is securely passworded for connectivity and for the router login too. Then have a separate router, (I have a Mac airport express) that will be open without a password to be a public WiFi for everyone to get on.

I want to be able to disburse the network equally without having too many clients on one router so that it doesn’t mess up and reboot. Cause today there was 60 connection on the nighthawk at lunch and it had to be rebooted. Would I be able to get a surfboard with 2 wans port and wire up the 2 routers from that? Cause the surfboard now is only 1 wan port.

Or should I just upgrade the nighthawk router to something much better to help handle the traffic and client connections? Cause the connection is business standards and is 1 internal in and 1 external out. Right.