Look, I really enjoy SH4, and the TMO mod, which it seems obvious you are using, but to be honest one thing that I've always found disappointing about the game was the brokenness of the campaign when it came to actually getting involved in the historical happenings of the war. I don't mean to throw cold water on your career, but even the basic TMO campaign can't overcome the fact that SH4 really has no dynamic campaign at all. That much being said, I have still managed to find quite a lot of action in many unexpected places, and I have yet to get into RSRDC (the Run Silent Run Deep Campaign), a mod which lurker hlb3 made specifically to address this problem. Here is what I've done: Tried to patrol in areas where it makes sense that convoys and TF's would show up. Along the southern coast of Honshu there are three areas which I have found to be good for this. The Bungo Strait, the Kii Strait, and the area surrounding the mouth of Tokyo Bay, which is actually called the Sagami-nada Sea. Now while not all of these areas will be productive hunting grounds all the time, they will yeild some action for you at some point. For these particular reasons these tend to be busy places: Bungo Strait allows access to the area near Hiroshima. There were IJN bases there near Hiroshima and Kure, as well as industrial centers. Kii Straits gives access to Osaka Bay, where one of the largest IJN bases was, at Kobe. Osaka is also an industrial center. I have run spec ops missions there and watched convoys come and go while I had to hide and remain undetected.
Finally the mouth of Tokyo Bay leads to a very busy place. Yokosuka, Koto, and Chiba were principle IJN bases there, and again, numerous industrial centers line the coast in there. Also make sure you are not using time compression so much that you overrun radio reports about TFs and convoys. Always check these out, they may be close enough for you to run an intercept and bag some sweet tonnage. I usually patrol at no more than 1024 TC on open sea, and no more that 512 in high traffic areas, and I keep a finger on a station command or the backspace key so I can drop out of TC quickly if a report comes in. Also, check out Suruga Bay, which is just to the west side of the peninsula from Sagami-nada. The game shows only two principle towns there, Shizouka and Yaizu, but there were others and the traffic I have found there has always been productive, though usually more for single merchants than convoys. I was in there one night and made contact on a Hiryu class fleet carrier running with a small escort (2 DD's), but it was going so fast I couldn't catch it. And on one occasion recently a large convoy steamed into that bay, and I was able to take 3 ships out of it. Be careful there though, its not very large, the coasts are protected by shore batteries, and there are patrol craft in the bay. General rule of thumb on shore batteries: Stay a minimum of 5 miles out if you suspect there are any. Use your compass to draw threat circles of that radius along dangerous coasts to help you keep your distance.