It doesn’t really take all that much to run 2Gb speeds if you’re not doing much with the traffic - any modernish processor in a small fanless box would work with 4-8GB ram. If you’re wanting to run Suricata, VPN, etc then you need more.

If you didn’t need 2.5 Gbps on the network, ace magician has a ryzen 5 mini that has a 2.5 Gbps and a 1 Gbps NIC for around 300 USD. Otherwise I guess you are looking for a new pc to put in fast cards.