Create your own custom router with an old PC.
OPNsense will run on just about anything, including a low-power Raspberry Pi single-board computer (SBC). I have an old Minisforum NucBox 2 collecting dust and decided to throw OPNsense on the system to act as a testbench of sorts for loading up plugins, configuring various parts of the firewall OS, and testing deployments before applying changes to my primary OPNsense device, which was a purchase specifically for running as a router.