As repeated here. The issue sounds to be the BFG. I am under the impression that the BFG may have different firmware to those currently being sold. Unfortunately the Firmware update process on these cards ain’t straight forward.
Worth a read here:
Various information about different Amiga hardware
gitlab.com
This is why I gave up with the BFG as it was too flaky even in my A4000D setup.
There are other alternatives such as the new Terriblefire 4060 and the Z3660 which have more FASTRAM. The Terriblefire has been tested in SCSI environments.
Worth a search in YouTube for reviews. Chris Edwards has started a series on the Z3660..