Is the published method of downgrading the u-boot version so OpenWRT can be flashed on UniFi not sufficient anymore?
[OpenWrt Wiki] Ubiquiti UniFi AP AC
The documented method requires no hardware surgery or console wizardry, and it's worked for me on newly purchased UniFi AC-LR and UniFi nanoHD, both with 2020 manufacturing date and coming with 4.0.x original firmware.