I would absolutely agree with Jason. There is no perfect answer. There are only options. And the best option is to use something that you or someone in your team already knows. If you know PHP. Why not use it to your advantage. If you know WP and are able to build it this way - good for you - just get started. Get users on the platform. Have people use it. And worry about the next step once there are users.

In the long run. I personally would build something on top of Node.JS with a front end framework of your choice (react.js, angular.js or ember.js are good options).

I hope this helps a little and keeps you motivated. In the beginning, it's all about actually bringing the idea to market. Actually launching and then see where you want to develop your own new solution.

And as Jason mentioned: even if you stick with WP as your MVP. Once you want to start developing it further, you can still use the WP REST API to read data from WP and start building a new interface around it.

Good luck and have fun on that journey!