Did you try FFSv1? I'm not sure, but I may be running v1. dumpfs(8) reports "format: FFSv1", although the sblock is FFSv2...That's next.
Using FFSv1 changed the verbosity of the panics, but little else: https://www.klos.com/~john/a4000_panic_pt4.jpg https://www.klos.com/~john/a4000_panic_pt5.jpg https://www.klos.com/~john/a4000_panic_pt6.jpg
Your panics seem related to disk operations.
I'm still puzzled about this, since it's happening on multiple types of drives on different busses, some with RDBs, some with simple disklabels.
I tested NetBSD 8 and NetBSD 7, and both exhibit the same issues. NetBSD 6 runs without issues, though.
I'm open to trying anything that might work or give more data. John