Problem solved. It was something to do with either fstab or drive permissions or the way it was unmounted. I played around with fstab options and created a problem during boot. Rather than try to fix it on my main machine I externally mounted it on a raspberry pi. I then used an old version of fstab (~6 months old) and completely rewrote it from scratch. At the same time raspberry pi was showing some folder permissions as no access for other than root. So I changed all those. Finally I’m not sure I unmounted it from the raspberry pi last time so I made sure I did it properly this time. The system is working great now.
I suspect the problem was in fstab since the other two don’t make sense, but then neither does fstab since I re-wrote it exactly as it was this morning (as far as I can tell). All in all a very strange problem for me - complete CLI access to everything but no GUI access. I did do a “zypper dup” as suggested, but even after 3 reboots that didn’t change anything. It was only after a fresh rewrite of fstab that its completely back to normal.
Thank you everyone for your help and suggestions - very much appreciated. Finally, apologies to dcurtisfra who suggested re-looking at fstab and I dismissed that - you were correct.