If you haven't already purchased your parts, have you considered using a Raspberry Pi Compute Module 4 and pairing that with a DFRobot router board? As of this writing, they're both in stock from Mouser (SC0695, DFR0767) or DigiKey (SC0695, DFR0767).
The CM4 I've linked is the cheapest "lite" version with 1 GB RAM and without any WiFi or eMMC. This is more than sufficient for routing purposes. The second NIC in the DFRobot board is connected directly to the CM4 via a PCIe lane instead of through a USB controller. I have this exact setup and it works great.