Ryzen 5 with a dual 10Gbps NIC, running Debian. Overkill for a router/firewall, but I run other services on the same hardware including an email stack, Podman containers, and small AI model for use within Home Assistant.

I wouldn't buy new hardware. Any modest machine built in the last decade would do. If possible, get a machine with an internal ATX power supply rather than an external brick, they tend to be more reliable.

If all you need is 1Gpbs and WiFi, OpenWrt on consumer hardware is probably enough though.

Use openWrt (https://openwrt.org), and use their hardware list to pick a consumer router with the feature set you need that can be flashed to use openWrt.