As my luck would have it, I fell foul of the SystemUI force close at boot last night and had to end up factory resetting from the boot loader.
I'm pretty sure it was not substratum but instead the app, System UI Tuner, which is aimed at Samsung devices and up until now has been fine as I set my tweaks in Nougat and just left as is. Until last night tgat is, when I toggled the Bluetooth icon and later rebooted.
Obviously there's no way to recover from that and to be sure it was nothing to do with Substratum, I uninstalled all overlays with ADB to no avail. So this confirmed it was something else, most likely System UI Tuner.
Staying away from that app from here onwards!
So much faff getting all banking apps and things back up o scratch. At least now I've hi hands on experience with Oreo's cloud restore feature which is pretty neat, and most of my app backup exports are on Drive anyway.
I'll also create a full phone backup with ADB every couple of weeks just to be safe.
Also funny thing is when I signed back into Google on the S8, the two step auth request popped up on my car's Android stereo as that's the only other Android device I was logged into. Just thought that was neat.