I agree with what everyone said and there are a lot of insightful things mentioned. Plenty of them I already did, but it's always hard to cover everything.
My biggest mistake must have been that I have taken the artist's concept art as proof of being able to do the work, whereas I most important work I needed from them was the digital art.
Do you think that artists prefer to do both concept art and the production art themselves, or if they are ok with working based on someone else's concept art? Especially if the series is longer. I know that my call as the creator of the game is final, but I still want to work in the best interest of everyone involved. Also, is it ok if the concept art style differs from what the production art style should look like?
Because if it would be ok to have a separate concept artist, then I could at least start working with someone on the concept art and have more things ready when searching for the production artist.
Ryan, I know we spoke a bit in private, but from your writing, it sounds like you have some experience working on games yourself, or at least have an interest in it.
Artists are definitely okay working off someone else's concept art. Think of all the fan art made these days of different anime/videogame characters (I see somebody already got to mentioning this! haha). It's also typical for artists who do commissions to draw off of clients' pre-existing concept art, for example, of their original characters (OCs). (With that said, it may get boring for a production artist if everything is already concepted out in your game. It's natural for some things to be missed and your production artist(s) will need to do concept thinking as well - (it's also not worth to concept out everything)).
It's also okay if the concept art style differs from the production art style. Things that should be apparent from character concept art are: age of the character (baby - child - teen - adult - etc), height/physique, impression/personality they give off, the design of their costume and/or weapons. The production style will take these different characteristics and try to match the "what" of what's already there, but might change the "how" (i.e. style - the production art may have a different style for drawing the eyes, but will still aim to communicate what the concept art shows).
However, very large differences in style can be problematic - don't get very western, realistic concept art made if the game will have a semi-realistic manga art style. It'd be a waste of effort and does make translating the concept more difficult.
Regarding searching for concept artists, you may be able to be less picky - since you don't need to hire a person who can execute the game's production art style. You just need to find a person that can make good concept art - images that are clear and communicate the concept of the character/environment/etc. The concept art, however, should still be made in some sort of manga style.
However, regarding developing the production art style and its style guide, you want to be the most picky here. The best fit here is a person good at both concept art and production art - they have technical and conceptual skills. They are going to have to try out a bunch of different looks for the style, so a bunch of rough and slightly-polished sketches - but they will also need to create examples of some production-level assets.
As for myself, I've done some concept art kind of stuff in the past. Nothing that professional, and nothing really for videogames. I am, however, acquaintances with a lot of aspiring and professional concept/game artists, and I keep up with some educational content coming from the industry. I wanted to work in the industry as well but it's too competitive for me. These days I'm just going with the flow, drawing for fun, and working on getting a new career started. I may be able to do some stuff for you, but I can't do too much.