Miniature nuclear reactors are very complex and high-risk entities. One might succeed in coming up with a functional reactor, but challenges in terms of cooling, reducing noise, maintaining crew safety, guaranteeing ship integrity, etc. These are complex areas on conventional submarines, but imagine the risk of nuclear radiation in a highly pressurized war machine! Just to get a functional and war-ready SSBN in today's time, we are talking about at least a couple of decade's worth of developmental work at sea!
I'd argue that the Europeans and Americans are seeing the pay-off of SSN and SSBN technology precisely because they've been working at it for a few decades, whereas the Chinese and Indians have only entered the field recently. Pakistan - even assuming the KPC-3 miniature reactor is real and can be made functional - would be an even later entrant. The lead time involved simply makes this an unfeasible venture; we need attack submarines (i.e. SSKs and SSPs) today to help defend our coastlines and coastal assets.
Which leads me to my next point ... SSPs aren't competitors to SSBNs. Rather, they operate different operating paradigms and serve different purposes. SSBNs and SSNs are for ocean going navies with cross-continental strategic interests. They require fast and long-range submarines to deploy ICBMs up the nose of an opponent.
SSPs are defensive entities meant for guarding nearby waters and shores. This is the Pakistan Navy's primary mission objective, and an agile, quiet and affordable system - i.e. SSPs - are the appropriate solution. Not SSBNs.