One of my favorite recent projects I've done was making a full wpa2 wifi access point with firewall out of a Raspberry Pi 3B. (Yes, I know the limitations of hardware. This was a "hold my beer" project.). If your old PC isn't too old and has virtualization instructions, you have a lot of fun with KVM for VMs and iSCSI for a SAN.
I agree with trying to move more of the network functionality out of the ISP provided device and into home servers. A simple and cheap access point[1] can provide all the L1/L2 you need. Then you can run dns and dhcp inside Linux, configure firewall rules, segment your network, etc. The sky is the limit!
Note: I reccomend a dedicated access point over hostapd because I had a lot of problems with it circa 3 years ago.
