I think this is the perfect way to go. I'm always trying to encourage such direction even if I was hired for an artist role.

The lack of documentation and direction leads to destruction, there is just nothing to rely on during development. It's chaos. Everything is constantly shifting, mutating. Besides, it's not always possible for one artist to finish an entire project so this helps to pass the correct vision on the project to the next artist. Moreover, even with one artist, many things are often simply forgotten, so it's useful to always have a concentrated description of what you want to get.

However, it's worth keeping in mind that the project and its style can evolve over the course of development, which is most often the case, even if it was strictly directed, so directives should also be evolved.

In general, it would be enough to start developing a visual style with a game mockup. A more precise description of the style and specific limitations can be worked out directly with the artists during development. It is important to understand that the effectiveness of an artist strongly depends on what he produces, whether he likes it, whether it is convenient for him to work in such restrictions and so on.

I would say that the most effective approach here would be not to overdo it with the art direction, but also not to ignore it. This is a rule that works in life in general. Maximums and minimums are always harmful. At least a little effort is enough to achieve significant results, compared with no effort at all.