12.1 bootblocks the last known good choice for older Dell h.w

a Dell (not the one with the cam problem I posted about as well today)
refuses to get. beyond BIOS device detection on any bootblocks after
12.1.

I've tried every .iso 12.2 through to FreeBSD-Current snapshots, and
they all display the same problem hanging on the bootable drive detect
phase. I don't like running newer kernels on old MBR bootblocks. Maybe
I'm wrong, but I always do gpart bootblocks update for a major FreeBSD
upgrade.

Is there some debug possible on the MBR boot stage? I see no path in,
to get more message.

I am loath to over-diagnose this, it is possible /boot/loader.conf is
working but the delay between runtime and output to console means I
"see" the problem as during MBR phase.

The last thing to happen is the first "/" of the text spandrel is
drawn. That's it.

There is a BIOS upgrade for this box. Its on a 2020 BIOS, the 2022
edition appears to be mitigations for CPU attacks, it doesn't mention
any boot time stability issues.

G