Australia has access to literal tons of uranium. If Oz decided it actually wanted nuclear weapons, they already have the capability to get there very quickly.
I would say aside from the EU, N. American, and East Asian nations you mentioned (those are incredibly unlikely, and even if they did, it'd likely be with AUKUS or NATO cooperation), the possibility of proliferation through sales from Russia or China are legitimate concerns. But at the moment, none of those really have the financial or technical ability, and it doesn't seem particularly likely. I'd also say China is a more likely proliferation threat, as Russia doesn't exactly have the cash to throw around building nuke boats for other nations unless they pay for it...and the ones who can pay for it are the ones least likely to do it.
For Japan and S. Korea, they are extremely wealthy nations with advanced technical capability and a robust nuclear power industry that lives under threat from 2 neighbors who don't love them and have nukes. If they didn't already have plans on the shelf to rapidly go nuclear, they'd be silly.
Also, it'll depend on how the reactors are fueled up. Reactors use fission poisons to moderate the reaction (and prevent it from doing what you would want a weapon to do). Separating those out would be yet another complication...and at that point, why not just enrich the uranium just sitting around in the country's mines?
