I've stumbled upon this device:
It is so cheap (~12 USD) for an IPQ4019 SoC, 128MiB ROM and 512MiB RAM. Looking at the table of hardware, I find that it is officially supported by OpenWrt 21.02 (device page).
The only remaining problem is, how to flash this device? Why it is supported by OpenWrt but no documentation is available? Does the guy who submits the code to support it just leave nothing for people to know how to get it working?