Do you know any DIY router solutions like http://espressobin.net/tech-spec/ or RaspberryPi?
A Raspberry Pi, while a great "compute" device for the price, has slow networking, even with an external Ethernet dongle. I personally have a couple ODROID devices that are similar, but couple their Ethernet adapter to the SoC in a way that they achieve much higher throughput.
ThePC Engines boards have impressed me with their reliability, build quality, and level of support. I run "server" OS on them in several applications. I've read, but haven't confirmed, that the CPU isn't quite sufficient for gigabit throughput with the full-on SQM ("CAKE") that some like to run.
Solid Run makes some interesting product. I haven't touched their current-generation machines myself.
A decent x86_64 motherboard at over 1 GHz is a choice, with an Intel dual-channel Ethernet card, assuming its bandwidth is supported by the PCIe slots on the card.
Lots of eBay/AliExpress/Taobao choices too