Just a matter of time and SKorea will also. After that, WMD may be next for both countries albeit a tougher goal for Japan
Japan is very close to nuclear capability. Absurdly so.. They still have 50 t of plutonim. They already make solid fuel rocket ICBMs..
I'd put money on the Japanese getting a boat in the water way before AUKUS. They're better off alone IMO
Japan would want to do it to show technical capability. They don't want to be reliant on anyone. I imagine every time they build a new sub generation, they draft up what a nuclear SSN would look like, cost it through, identify duel use techs, etc. They would be benchmarking them as much as they can and the best way to do that is to basically do design work.
Both have significant nuclear and submarine experience. They could certainly do it. But the SSN may look different, I wouldn't be surprised if they favour low speed performance, completely silent reactors, etc. I don't think they want an AUKUS or French style sub anyway. They don't have to travel far to find targets.