Their involvement with Australia's requirement has probably shifted their future design specs. Part of the issue of the Japanese subs was that they had built them for the Japanese CONOPS very specifically, so things that needed to be very good and modern for that, were, but things like the periscope, which they didn't utilise in the same way, were old school.
Sizing for Australian crew isn't really the issue, the height difference between the average Australian and the average Japanese is ~3cm. Subs are always small and cramped, but japanese subs and Japan as a culture probably adopt an even more compact personal space approach. As someone who is 6'7 and been through Collins and Oberons, more room would always be better, but on a sub changing deck height is essentially impossible.. One of the benefits of opening women up to submarine service is their generally smaller dimensions.
But long range patrol and endurance is. But then again, a lot of that was based around being based in Australia and for use during peacetime or for long range offensive operations. That may not longer be the case. While we wouldn't want to give up all long range capability, if it is the only subs we could quickly acquire... and if they were forward based, or just used for continental protection within our EEZ....
Japan has a neat trick where it tends to slightly overbuild to ensure it has a constant production line of ships, so it can shuffle its service life/decommissioning generally if required to make boats/ships available such as what is proposed with Mogami. This is how Japan rapidly increased the number of operational submarines, by very slightly increasing production rates, but also increasing service lives. Because the actual platform material cost, isn't actually the big part of the over capability cost.
Japan can quickly and inexpensively increase its force of diesel-electric combat submarines from 22 to at least 28, if it stops prematurely retiring them. That would provide more of the one category of warship that ...
www.aspistrategist.org.au
Building a boat a year, Tokyo adjusts fleet size by adjusting ship retirement age
asia.nikkei.com
Other options include acquiring the Gotland class. The Blekinge class you mention above is expected to be delivered in 2027-8. Gotland is essentially a mini-collins structurally. These hulls may be useful to acquire for spares, training, de-risking developments, de-risking new yards etc. Even if they just operate out FBE or FBW for coastal defence and ASW training duties. It may also be attractive to acquire some swedish submariners, who want to travel with the platform over to Australia, rather than retrain for the new gen subs. Training and layout would mostly carry over. It may allow those who are looking for a different type of submarine service to stay in the service. Do more like a two week deployment from Sydney to train regional surface combatants. I've mentioned the Singapore Archer class, which are also of a swedish design and have had some Collins class improvements applied. That would be an additional 5 operational submarines, able to be acquired for essentially scrap value off allies, that share a huge commonality with Collins. Sweden also has the Sodermanland submarine, so you could bump that up to 6 hulls... They are smaller crews, so the crewing and collins commonality would go a long way.
But these would be coastal submarines. They are not blue water subs. But as we have seen, there may be value in having something that can operate in our own EEZ and train our surface ASW capability.
So again, while an idea, we would have to be pretty desperate to put so much energy and effort in getting these 1980's relics into service with the RAN. I would imagine Japan would readily make available subs to Australia, particularly if they were based in Japan on rotation. In trying to expand their submarine service, they too will find it difficult. But extending the service life of a submarine also generally means, extending the service of those crewing it. Their model is one we should definitely be looking at.