JuicyCoalaB
You can disable the router’s wifi frequencies so that you can use your Nighthawk as your Access Point.
If you want to continue to use it as a modem, then you’ll need a new router with at least 1 2.5 gbps WAN and 1 2.5 gbps LAN ports, where you’ll connect the 2.5 gbps WAN to the XB7’s 2.5 gbps LAN port, and your new 2.5 gbps switch to the 2.5 gbps LAN port of your new router. And that will cost more money. You said the cheapest way, thus my suggestion above.
You can maximize > 1gbps bandwidth by wiring your devices with at least 2.5 gbps NIC card (i.e., PC) back to the 2.5 gbps port of your XB7 in modem/router mode. To expand the ports, you can buy at least a 2.5 gbps unmanaged switch, plug it to the lone 2.5 gbps port, and then plug your PC to the new switch. That’s the cheapest way.
Your Netgear router only has 1gbps ports, so to use them, you can plug it directly to one of the XB7’s gigabit ports, and make sure it’s in Access Point mode. Plugging it to the 2.5 gbps port of the switch or the XB7 doesn’t gain you anything beyond 1 gbps.
The only risky stuff I do is download pirated games on my desktop and very, very rarely watch corn on the laptop.
Downloading illegitimate software and installing them onto your PC will bypass all the security set by your network devices, as the networking devices doesn’t have the capability to scan/check if a downloaded file contains malware or not. A good anti-virus software may be able to sniff-out a malware from your downloaded file; therefore, building your own router doesn’t really help you in any way.
Might be good to cross post this on r/eero