There are some routers with OEM fork of OpenWRT that allow you to flash stock openwrt without any obstacles and by easily dropping the image blob in the web UI - just easy as upgrading the firmware from stock openwrt.

Granted, not exactly what you wished for, but pretty close.

The time to do this is <5 minutes. One such router I personally know of is GL.iNet GL-MT6000: https://openwrt.org/toh/gl.inet/gl-mt6000