Good freelancers are expensive because they know the value they provide. In this sort of situation you tend to get what you pay for.

I don't know why you think they won't stick around long-term. I've seen many freelancers maintain relationships with clients for a decade or more - not necessarily full-time as if they were employees of course but with a genuine connection and coming back to do more work with the same client from time to time. If you want to retain access to the knowledge and insights of your early developers but also want to bring things more in-house - often because you don't need an all-star team to do every little thing once the system is in production and hiring a more mixed team is sensible financially - then this kind of occasional recurring contact can work well for everyone.

That's fine; this would be an experiment, after all. If it goes well, those freelancers can be tasked to help hire a more permanent team, and/or be offered permanent positions, if they're interested.

It depends! I've probably stuck around longer term more often than not. You can also write contracts that enforce the term if you want. Agree on things like this up front.