I am a writer and artist. I know both processes well. So I can say that writing is faster than drawing. I am not a very fast writer, but if I have plan, I can write a chapter (1500-2000 words) in 1-2 working days. One illustration of this chapter will require more days or even weeks. Especially a lot of time is taken away by battles, battle scenes, complex background with buildings or interior.
Character design is a very complex process. You can not immediately draw the finished character. Preparatory work takes time. These are sketches, references, design options, color options, etc. If the artist can sit down and in a couple of hours draw a sketch from the scratch with a interesting character design, then this is a very experienced concept artist who gets a lot of money for their work.
If in exchange you propose only to write a few text for the artist, this will be an unfair deal. You want more than just a cover or some illustrations.
What is the expected final product? How will you monetize your project? If you have income, what will the artist get? Who will have the rights to all these illustrations? There are a lot of questions.
And one more aspect. This is your project, your world and characters. This is not co-creation, when two people work on the same project and can offer ideas on equal rights.
I think it is difficult to find an artist who does not have his own ideas for implementation. This is motivation. To work for free on a large long project, there must be a strong motivation.
I wish you find a suitable partner. But it seems to me that only a very inexperienced artist can agree on current conditions. Just because they does not represent the amount of work.
You are more likely to find a partner if you offer to make a comic for your project and share the possible income.
P. S. Sorry for mistakes. My English is poor. And it's hard for me to write such complex texts.