For something with hardware offload, get a Ubiquiti EdgeRouter. I run one at home, it's debian based, I have lots of tools I've written in Go compiled and running on it for various purposes and you can install debian packages for things you need.

The other option I've heard good things about are the PCEngines devices. They don't, as far as I'm aware, have hardware offload, so make sure their performance suits, but they use OSS U-Boot and you install the OS of your choice. It's one of the most open devices in a router form factor I've come across.

Depending on the number of ports you need, you could also use one of the Jetway devices. They make them with varying numbers of ports as SBCs in a case and you add RAM/M.2 SSD. I got a Celeron one with 2 ports and run it as a Suricata IPS. It performed just fine with my 100M pipe.