Those missiles - i.e., DF-3A - that China sold to Saudi Arabia in the 1980s are the least sophisticated military hardware it ever produced. It is notoriously inaccurate. China in 1980s would have sold its military hardware at almost any cost to anyone, so long it returned financial or political profit. Also, and I am saying this respectfully, your suggestion that China is "more than willing to export" n-subs [to Pakistan] is oversimplified. While indeed there were rumours of Pakistan considering leasing Type-91SSN, I do not think that there was much seriousness in that contemplation. Besides, the submarine was a floating radioactive/cancerous death trap for sailers anyway. On the other hand, selling or leasing Type-93 SSN or Type-94 SSBN is out of question for several reasons:
1. Political cost would be too high, although since much reduced because of the AUKUS n-sub deal (political cost remains high for Pakistan though).
2. Resource constraint, especially financial and infrastructural, of Pakistan/PN.
3. Foremost of all, PLAN's requirements come first and they have not been met yet.