Recently OpenWRT released version
20.02 and it’s great!
The Wiki instructions for flashing Ubiquiti AP AC devices (including Pro, Lite and LR versions) has a number of options. One uses
tftp but requires a specific version of the stock firmware, others use
mtd flashing tools (either from OpenWRT or stock) flashing tools, and another via serial.
I chose option 2, using mtd-tools from OpenWRT, as I think it’s the most simple. However, it did require a few extra tweaks for
20.02 due to library errors.