I'm now thinking it may be official that I have solved my problems... I've made it for about an entire Sim Week and a few days without a single crash! I'm like 7 chapters ahead of updates right now but I'll keep them coming steadily.
Next weekend I'll be trying UC in the full unedited world and see if it copes still and then move the legacy to a new file if it does.
I am so pleased and, I love this Legacy so much more than I have liked a Game before. I almost don't have a Sims life outside of that file now.