Just a word on what it takes to field a submarine arm. The RN kept 3 subs in Australia until 1969 to train the RAN in ASW, the RAN ordered 4 Oberons to replace this ASW training capability. The RAN planned to acquire another 4 Oberons to boost its submarine arm from an ASW training arm into an offensive force, but in the end only got 2 Oberons and a further batch of A4 Skyhawks for the FAA. 4-6-8 diesel submarines is of course dependent on Australias unique geographical and alliance circumstances , the differences are only in degree rather than order of magnitude. If Australia needs 4 subs for ASW training and 8 for offensive the a NATO member likely needs 3 and 6.
Canada looked at SSNs a couple of times, 1958-60 and 1987-88. However Canada tightarsed the whole process, they dropped the SSN idea, looked at expensive US Barbel class SS, then 6 cheaper Oberons then 3 Oberons the delayed the buy because the British didn't offset the purchase in Canada fast enough. The 80s Canada class SSNs wasn't much better, it was opposed by the US for starters who had the right to do so, and again the Canadians weren't really keen on C$8 billion on subs. So Canada is out.
Nuclear was mentioned in the Collins class in the 80s, but nuclear in Australia was so far off the radar that it sounded ludicrous, same with Collins replacement. Indeed until the AUKUS announcement the merest suggestion of nuclear submarines would be laughable. However I think that with a different nuclear history, if Australia got a nuclear power reactor in the 60s, the idea of a nuclear submarine wouldn't be quite so laughable even if it was rejected on cost and international political grounds.
As for others. In the 60s an Oberon cost 3 million pounds, the first 2 British production SSNs cost 25 and 21 million pounds. That is an absolute fuckload of money, 2/3 the cost of CVA01, which was cancelled for being too expensive. Which countries have that sort of money, and the requirement for such naval power?