It's easy to criticize the decisive battle mind set but honestly it was their best shot. Win a decisive battle or a series of them early on and hope (yes hope) the Allies decide the western Pacific isn't worth fighting for.
Their best shot was not going to war in the manner they did in the first place. As my research when living and working in Japan showed me, there were officers in the IJN who were far from stupid and knew a quick war was not certain, and were indeed mindful of possible US reactions to moves they could make. Some were not happy at all at the prospect of going to war with the US as they knew they would lose and
why they would lose. However, most of the leadership did not wish to hear of such defeatist talk. Large parts of the IJN doctrine worked on the assumption the opposing force would follow what they predicted for it, so it was flawed from the start. Warnings were ignored over a long time, when there could have been an opportunity to address various deficiencies in the fleet that were known about well before the war even started.
Going to war, you need the best plans available and to be able to listen to various ideas put forward. This was a failure of listening to warnings and learning exercises that did not match up with their biases. If it contradicted their assumptions, it was dismissed, and this is prewar stuff. We all know the examples of Midway wargaming when Nagumo's carriers suffered losses, and these being dismissed as well, but this was a characteristic which was entrenched and existed before the war, and it was a dangerous one.
If it was just about after the war as in OTL, then you have a point their best shot was a short and decisive war, that was indeed their only hope and what they were aiming for. I get what you are saying, yes, it is indeed easy to criticise and we do have hindsight. Trouble is it's still easy because they could have done something about it much earlier due to awareness of the flaws in the doctrine, and various IJN officers found it easy enough to criticise it themselves, but hard to get listened to. However, once again they didn't use their submarine fleet even in
its intended role efficiently and properly. The fact they had been warned a number of times by officers and analysis in their own exercises what could be a real danger and then dismissed it because it wasn't convenient is the real issue.
Back to the question about German U-Boats, we can see that even if they get them, there's not going to be much change in the outcome, even if they do somehow alter their doctrine and use them to attack SLOCs as was shown as at least somewhat feasible by reports from their own submarine commanders. It would be too little, too late. The worst that happens for the US is somewhat of a slowdown in the their advance, but it won't delay matters by a large margin.
However, if people desire more results than OTL, then at least their submarine force would have something more to show for it, even if it winds up not that much, and that's the point that's trying to be got at and seems to be what the topic of this thread is asking about. But we don't just need a POD well back in the 30s, doctrinal change is required as well if more than that is desired. And if they can manage to listen and change one part of doctrine, well, it may not be impossible other elements may perhaps change too. It's quite an ask though.
People can have deep disappointment in the submarine arm's performance (as happened) or a little less disappointment about how it was used. It's not like the focus of attacking warships actually did that much to change the outcome for Japan, apart from a few notable successes.
Even the employment of the submarine role in the doctrine that was expected of it largely failed.
Phew. Doing too much writing today. That's not normal. I think I'll relax with some more Ken Boon telly stuff.
Sargon