WAN <-> VPN Provider <-> WAN <-> Cable modem <-> Pi-Hole/VPN <-> Home router <-> PC's, phone, etc

added VPN provider

So you'll need to set up a VPN service on your Pi, then set up iptables for allowing traffic to flow through the Pi, and use DHCP to set the Pi as your gateway. You'll also need to point the Pi to your home router as a gateway. Note that your overall throughput will be limited to the NIC on your Pi. The specifics here depend on your VPN vendor.