There are a couple ways to do this.
Some model routers offer a guest network mode which isolates the devices on two subnets. Unfortunately the vast majority of these are wifi guest nets. So everything on your non server sude would be connected by either by wifi or a bridge. If you get the right model devices and configure then correctly and place the bridges close to one another you can get well over 1gbps. Its a bit of a redneck fix but...
Dmz and port forwarding is option 2.
Option 3 is isolate your personal pcs behind multiple layers. Use an active switch and assign one router one subnet and a second router a second subnet. You could use bitdefender box 2 to protect your personal pcs. It looks for people attacking or port scanning your network.
https://www.pluralsight.com/blog/it...to-organize-your-network-into-smaller-subnets
Pfsense with snort is another good option. Snort is an intrusion detection package. Problem is it has a hell of a learning curve. Ots also very jard to configure if games require upnp. Youll have to jump through multiple hoops and it wont work if two people in your house decide to play something like counter striie at tye same time.
Please be aware without proper traffic shaping at the appropriate point that your family's bandwidth may slow down your server. If your server needs gaurenteed priority youll need professional hardware. Consumer grade qos wont cut it.