Replying to my own post from a year ago, I'm not sure why Xiaomi Mi 4A Gigabit Edition hasn't been posted here sooner. It pretty much ticks all the boxes from my previous post, except the USB port. Probably because it initially required opening the device, but it is now flashable using software-only exploit OpenWRTInvasion.
Dual band WiFi, 802.11ac, 2x2 MiMo, Mediatek MT7621, 128 MB RAM, 16 MB flash, 3 Gigabit ports, quite easily flashable, hardware offload works, negligible power consumption (4-5W) and costs less than 30$ shipped worldwide! I have bought it for an equivalent of 29 USD in a physical store in my country!