Well I've fixed my instability issue. The root cause was the same as https://forum.netgate.com/topic/144636/sg-1100-intermittent-reboots, and it was power related. The fix was to replace the 470uf/16V capacitor (component EC1 on the schematic) sat immediately behind the 12V DC jack which under testing leaked under certain temperature conditions (variations). As such the board is now stable and has been running OpenWRT 18.06.2 for 24 hours with memtester and stress running continuous during that