Yeah the first one was about the setting and new visual tech. No one rags on Star Wars: A New Hope because it followed the basic hero's journey with a story that ripped off Seven Samurai.
Now that the setting is set and they've had a long time to work on it while the emphasis will still be on the visuals I'd imagine the 'story' will find it's own place and become much bigger than a basic retelling of something that's been done before plenty of times.
An established setting and a lot of time to work on a bigger, better story didn't get us much in Star Wars' case, though. We got the flaky prequels (that were bad partly because Lucas tried to tell a larger story), then a basic retelling of the first film in The Force Awakens (exactly what you imply shouldn't have happened), then The Last Jedi. If anything, Star Wars proves that following up a phenomenon is extremely difficult, even when the original was simple and derivative. You might think that that latter point means that there's lots of potential for improvement, but, in the film world, that often makes it even harder because there's not enough to build off of. In such cases, sequels often end up appearing either too derivative (like in TPM and TFA) or too different (like AotC, RotS and TLJ, so that fans think that they don't "feel like" the originals).
I wouldn't put it past Cameron to prove me wrong, but I would not be surprised if the first Avatar sequel is essentially a retelling of the first film, much like TPM and, especially, TFA were similar to ANH, and if the later sequels are hardly more original and more capturing of viewers' imaginations. They'll all be commercial hits, but for the visuals and the events that they'll be. I don't anticipate the stories being well received, no matter how much time they've had to work on them. After all, Cameron wrote the first treatment for Avatar in 1994, 15 years before it was finally released to theaters, and he didn't write or direct anything major between Titanic in 1997 and Avatar in 2009. That's a lot of time to tweak and improve the script, yet it still ended up as simple and derivative as it did, and that was just one script to work on, not four. In other words, I don't think that having a lot of time to work on these films should be much comfort that they'll be an improvement in the story department.