Large Database migration into WP

Howdy, all! I have a client who has a custom PHP site with approximately 40k articles and a custom built function for accepting and reviewing new articles. They want to move the existing articles into WP and then on an ongoing basis, migrate new essays in the same manner (they need to keep this custom built process).
I’m thinking as long as the schemas are well mapped, it should just be a matter of dumping the existing db into a format WP can recognize and then importing it into wp. My questions are:

Does this sound crazy? Am I missing a lot?

Any tools people would suggest?

Can anyone direct me to in-depth articles about the wordpress schema?

Anyone know of case studies where something like this has been done, or know any developers that have done it?

Thanks!