How can I quit my H-1B job and start a business with pending EB-5 I-485 and I-526 forms?

Your understanding is correct. It is usually recommended to keep maintaining your current status if possible, but if you really want to use the EAD you can use it.

If your I-526 case is strong, meaning the source of funds is clean and the project has created the requisite number of jobs, then you can likely take a chance if you are not worried about maintaining lawful underlying nonimmigrant status, which would provide a safety net if the adjustment or I-526 petition were denied.

You have listed the one possible consequence which is you will be without status should the I-485 be denied. There are no other issues with quitting your H-1B job and using your EAD card to pursue your own business.

That is the main consequence.