The following patch should add Archer A7 V5 support and allow flashing from the factory firmware:
https://gist.github.com/dword1511/b0ead3a1a8f3bfae35d710b4a17b4ce8
I had to create a separate board file (mach-archer-a7-v5.c) due to different art and default-mac locations.
However, in the middle of the process, I bricked my router and somehow restored the factory firmware. Some of my factory partitions are empty. Although factory firmware runs normally and can be flashed into OpenWRT, I am not 100% sure that it would work on a clean, out-of-box A7.
Factory firmware boot log (and my botched first attempt):
https://gist.github.com/dword1511/1a2b750be9f3bd8eae4da0faff7ebebb