Yeah, it would make a beefy router for sure. Wouldn’t be very power-efficient, but would handle the job well.
Outside of that, you’re most limited by the 512 MB RAM. Adding a larger drive would be an easy/cheap upgrade (though it may be SATA II speeds or possibly SATA I).
If you use OpenWRT for your router OS, you can also install AdGuard and get a bit more use out of it.
If you can add an HBA for better SATA speeds, and have room in the case, it might make a halfway decent NAS (or backup NAS).