Note: Not at all an emacs expert, but know some stuff about trying to parallelise a long-standing language ( GAP -- www.gap-system.org , 30 year old maths language).

I'd strongly recommend something like Javascript's "web workers", where each thread lives in an almost entirely independent memory space, with sharing limited to explicitly named buffers.

The problem with traditional "everything shared by default" threading is even when you get the basic core working, we found throughout the system there were buffers and caches, which all needed making thread-safe. Even after that, every library needed some fixes -- and while these were often simple these libraries may not have had updates in years.