Don't start yet
One of the most typical mistakes in inexperienced writers, which I still make, is starting to write the story too early, before it is refined enough.
In my case, the plot isn't usually the big problem, it's coherent and it has several hours of planning behind it. But other aspects, such as the development of the characters, are still difficult for me. The consequences are the expected ones, characters that aren't as authentic as they should, characters that work more or less, but aren't at the level they should be.
The problem with writing too early is that at the beginning everything is very easy. With the story just beginning to unfold, there are hardly any restrictions. But they will appear as the story evolves, the space to overcome these problems is becoming smaller and smaller, leading in most cases to blockages, plot gaps, etc..
But when the previous work has been done, the story develops taking into account where we want to go, the characters act as we have defined. It doesn't guarantee to avoid complicated situations, but even so, we'll always know (because we've designed it in the plot) where we want to get to.
That's going to be my big goal in the coming months, when I will start with the third part of "The Factory of Gods". A lot of previous work, especially in the development of the characters.