YES! I’ve figured out what this was. The megascans trees come in two folders - simple wind & pivot painter. For whatever reason the simple wind trees don’t work with nanite and wind enabled - at least for me. However, the pivot painter trees work fine! Just as an aside, megascans recommend pivot painter for high fidelity foreground and simple wind only for background.
Anyway, so I thought I was in for a painstaking task of switching all my simple wind trees to pivot painter - because the ‘replace with…’ option in foliage mode always crashed my machine in 5.1. But what’s really cool is that unreal has fixed this issue in 5.2 and I could quickly swap them out. Everything now works as expected. So if anyone ever runs into this - just switch to the pivot painter trees. Cheers
EDIT: Only just seen above solution which is better as PP trees are quite heavy.