They definitely won't play ball with a platform attempting to integrate all of them. That's just a risk; there's no upside. It's possible that none of them would welcome a "platform" at all.

But survey the landscape of EHR vendors. There are a few unique positions of strength out there. Each aspect of uniqueness defines a hypothetical market. The mere existence of some of those markets would develop an asymmetry that simultaneously penalizes competitors and solves a big problem for the partner. Build the spark of the market and you have a significant ally with a strong incentive to nurture your business.

Idea quality indicator: you want the existing player aligned with you, not buying your company (then killing it). They should need you externally.

I'm building a business on one of these hypothetical markets. I made my choice based on my personal capability and fit with the details of the problem. But there are a huge number of different ways to improve both the quality and efficiency of healthcare. (You don't get to solve big problems if there are no big problems.)