That's really helpful. I've started checking through the items.
I was having my system contstantly restart when waking from overnight sleep. If I booted it from a backup and then restarted from the boot volume, it was OK. I restored it from a backup but that was missing some updates. I updated to BIOS 3504, disabling the keyboard/mouse emulation as advised by blacktea and checked the config.plist for obvious errors. Then I got the latest MultiBeast 9.2.0 and ran through the config and installed it, and made sure USBInjectAll.kext was at 6.2.0 (it had dropped back from the restore).
Now the system seems quite stable — 10.12.6, iMac18,3 which is very close match to my system, and the macOS Native CPU/IGPU Power Management. I slept it overnight and it woke OK this morning. The only issue was that Safari had blank boxes in the toolbar where the action icons should be. All I could think of as changed was the BIOS settings for Quick Sync which I had done. I undid those and Safari's toolbar is now OK. So I'm not going to change anything for a few days and make sure it stays stable.
I'm feeling close to being able to give my mid-2012 iMac to my grandson and rely completely on this system.