Overall concept sounds like a good idea to me, but I see a potential middle ground between “only initially specify a subset” and “bulk copy & paste from the PEP specification sections”: adopt the approach of building out a self-consistent specification from an initial core, but include a stub section with relevant PEP references for every topic not yet covered in clarified prose (this approach could be used for both the implementor facing formal spec and the user facing docs).
Expanding on each such section might start with a PR based on the text in the relevant PEPs, but could be tidied up and better integrated with the rest of the text in the initial PR review.