Suspend / resume on Mac Pro 2008 (3,1)

Hi all,

I know this is a tricky one to solve but I'd be interested to know how I can debug this further, or at a minimum, just stop the machine from suspending on it's own as it's impossible to recover from it once it happens without a power cycle.

Disablig ACPI completly with a boot hint stops the machine from booting, so that's a no no.

After logging in as root after start up (no X11 etc), using:

acpiconf -s 1

causes the machine to go through the motions of suspending (hard drives spin down, USB devices go off), but it eventually locks up with:

ahcich0: stopping AHCI engine failed
ahcich1: stopping AHCI engine failed
ahcich5: stopping AHCI engine failed
ACPI Error: AE_NO_HARDWARE_RESPONSE, Returned by Handler for [EmbeddedControl] (20181003/evregion-424)
ACPI Error: Method parse/execution failed \PTS, AE_NO_HARDWARE_RESPONSE (20181003/psparse-677)
acpi0: AcpiEnterSleepStatePrep failed - AE_NO_HARDWARE_RESPONSE
ahcich4: stopping AHCI engine failed

The monitor stays on at this point, as does the power LED (instead of fading in / out when suspended fully), and the machine is effectively locked up.

At this point I'd be happy with a path to just stop the machine suspending on it's own, as going away from the computer and coming back again after a while means a power cycle to get it going again.

I'd like to keep this old beast of a computer going for a few more years if possible, and I'm getting pretty tired of Apple's constant software upgrade push (most of which is buggy / annoying / unsupported on my machine anyway), so any help would be much appreciated.

Again, I know this is tricky to debug, but I'm happy to put in some effort to try and work out what is going on to improve the situation if I can. I'd really like to stick with FreeBSD going forwards as there's a lot to like about it (it's so peaceful compared to macOS), I just need to iron out a few blockers (such as this one).