I have not played that many games into 1943 but I've learned a few lessons playing the Nips;
Don't waste your carriers in futile attacks that will only give your opponent a lot of victory points. Only use them if a 'true' opportunity can be found! Transfering the carrier aircraft to land bases might be more productive.
Conserving your pilots is extremely important - you're most likely never going to get this experienced replacements later. If your cv's are sunk with their airgroups aboard, it's a double tragedy! At least let them have the opportunity of transfering to a nearby base.
I know that a lot of players don't build Lunga and other bases because they say that it will only aid the Allies when they occupy them. I don't think that way - US engineers create bases faster than you can say WTF anyway.... So I create a network of bases that are mutually supportive.
Getting early warnings is vital, so forcing your opponent to take out bases that threaten him will give you a clue to what's going on.
Putting all your base forces in forward bases is usually wasteful. Remember that the US heavy and medium bombers will ruin your day no matter what. Getting 100 ac's stuck on a closed base is really bad for you. Put in a lot of air defenses and withdraw your aircrafts and base forces behind e.g. Lunga and PM. These are initially out of enemy bombing range and can be used to mount counter strikes.
Having huge numbers of aircrafts is a must - never send them in piecemeal....
Try to save your Betties and Nells and release them when they really can make an impact. Having 1-200 bombers hit your invasion force isn't very funny.
2-300 Zeros do bite hard if they're not manned by green pilots and worn out, so try to rest them and transfer them to the rear when their fatigue/morale drops too low.
Rotating your troops is also vital, this is a bit strenous but pays off. Having 20k fatigued troops on Lunga doesn't do the trick. Watch your base forces as well, when to many support units are out of action, send them back to Truk for a month or so.
Mine vital bases heavily, and then I mean heavily...... If you know that the enemy has to cross certain shallow water hexes, then mine them as well.
Placing surface combat tf's on react in bases 4-6 hexes from potential invasion hexes also pays off. Small tf's with 1 cl and 4 dd's can really ruin the day if they achieve surprise.
Let the enemy come to you and fight on your terms, never react to his initiative but let him play it out until the situation suits you. Then you strike - hard!
It will take time to capture a base fortified up to lvl 9 and garrisoned with 10-20k troops. I would never reinforce the base if I'm not able to protect my transports. So you should control the air/sea before bringing in reinforcements. If the enemy lands 40k troops, you will have to bring in another 30-50k men to save the base. If you can't achieve this - don't.
You should never forget to store enough fuel and supplies in your main bases at Shortland and Rabaul. You burn a lot of them when the going gets tough...
PM is a problem indeed, it is very difficult to effectively defend it. Again it will be important to delay the invasion force long enough to enable your forces to mount possible counter attacks. Building up enough bases behind and above PM makes this effort simpler.
If you can't throw in a wildcard now and then, you will loose. So do what the enemy least expect. When he attacks one place, he will be weak another! All invasions got long flanks... Make your opponent paranoid, this will drain forces and resources away from the front and make it easier to make an impact.
Patience is often one of the first casualties of war, things don't always have to happen as fast as possible. Use what time you got and try to create a plan so cunning that you could put a tail on it and call it a weasel
Others from that you should train your reinforcement pilots before sending them into combat. I don't like committing them until they reach at least 60 exp.
Well, just a few thoughts - maybe you can use a few of them
Good luck!