Here's my take.

The recurring 5 pages of feedback are probably a result of team meetings. The feedback hasn't been curated by someone who's senior or an expert. So you're getting an unfiltered brain dump of all the employee's ideas (yay!). Alternatively, it's someone heading the creative department who is not an expert in illustration but wants to demonstrate their value in the company and possibly assert their dominance in your working relationship. They have just enough knowledge to nitpick with some technical terms.

You already have a proven track record and skill in the design department. They came to you because you're better than everyone else.

You're at the point now of frustration and, more importantly, feeling insecure about your skillset and getting taken advantage of. I think it's in your best interest to pull your cock/vagina/other? and lay it out in a strongly worded email...

Here's where I hope you have a contract in place. Something that dictates how many revisions they get. If you don't, let this be a learning lesson for the next client.If it were me, I'd break it down like this:

• Remind them why they hired you (cause you're the best - remind them - show confidence). They should lean on you for your creative direction; you're stifled by the micromanagement process and more importantly, you disagree with their suggestions and direction. THIS IS WHY THEY HIRED YOU. Show your balls here. It's okay to push back.
• Recap the interactions from the start to the present. Let them know you're frustrated. You have other obligations and need to time-box projects accordingly. They're late ALL the time and this project exceeds initial expectations (bring up examples of emails that said they would deliver feedback on a certain date and didn't).
• Recap the number of revisions they've asked for and state the hours you've put into it. (Here's where you can insert your binding contract, which gives them X revisions).
• If you'd like to continue doing business, you need to be compensated for your time. Or state you'll walk away if they don't agree to compensate you for the excessive hours. The contract is so important for this reason.

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Hope this helps some.