Xiaomi Mi Router 3 (mt7620 + mt7612e)

I wonder if anyone could build OpenWrt for mt7620 + mt7612e combo (the first chip is SoC and second is 5Ghz PHY), I'm asking this here since I found mt7620 + mt7610e combo in releases, and I think it shouldn't be hard to port this for mt7612e to ... (MediaTek mt7610e doesn't support MIMO unlike mt7612e)

There is a build with instructions in openwrt websites but after flashing kernel and firmware it stucks in an infinite loop of booting ...

IIRC, the NAND driver is/was a blocker for this device. It is very similar to the Xiaomi MiWiFi Mini.

If you want to install custom firmware without hardware modification, try using X-Wrt, they integrated the vendor NAND driver. However, I think they use the proprietary wireless driver and not the open source mt76 driver.

If you want to install vanilla OpenWrt, I wrote the SPI flash modification instructions here (click).
Doing it this way essentially converts your router into an oversized MiWiFi Mini - you can install and run the bootloaders and builds for Mi WiFi mini.