More often, I find it easier to work with developers when I let them hand me a very clean and simple finished product in the way they like things done. The best pros can usually dump out a simple interactive website within a week or less, and they are happy to walk you through how to use it, and how to edit things. They've done it a million times. This is how they like to finish a project, so they can get paid and have a sense of accomplishment.

If the developer succeeded in delivering the clean and simple (though limited) system, spend some time attempting some of the redesign features on your own until you are ready to hit the wall... call the developer back a few weeks later and ask if you can hire them for consultation. Often times they'll look at the mess you made and ask why you did it that way, and they'll have a simpler solution they can build if they are wanting to expand their contract.

They might even tell you the base system was never designed for what you had in mind to do with it. This is the time to re-evaluate if you want to work them again. Are they going to con you for more time and money, or do you trust they are motivated to deliver what you need?

Keep it simple. Give them one simple short-term contract goal. Set a deadline. Not finished? No pay? The contract stipulates what the capabilities of the final system needs to be able to perform. Multi-month jobs on a project with many challenging requirements tend to overwhelm people when they are working solo. There is no job stress that compares to the kind when your manager suddenly reveals new insights of what the final results should look like after you have spent half of your available hours doing it the wrong way.

Best of luck.