"If a tree falls in a forest and no one is around to hear it, does it make a sound?"

You can spend countless hours making improvements but without a user base, you have no idea if they are needed or wanted. Build your MVP, and whilst you're building it, shout about it. Tell everyone. Nobody is going to steal your idea.

Marketing should start from day one and in my opinion be bigger than a 50% split. More like 70% marketing. This will let you streamline your development time and only build things that will benefit your users, and ultimately your revenue stream.

Good luck with your product :)