So ... you'd be best to just do the simple BM downgrade to a non-SSH lockout version, then install braiins
A 2019 firmware of Bitmain doesn't permit you to downgrade to an earlier version, you are forced to remove the signature check to load any other firmware than a 2019 firmware of Bitmain.
And I never installed the Asicseer firmware, I just removed the signature check with their installer. You can do this without installing firmware.