"Now that we're figuring out the technology stack for this, I don't want to do work that is going to have to be redone later."

You're worrying about problems that you don't yet have and at scales you may never reach. I'd say stick with what you know and see if you can get initial traction with your users.

If your data is highly relational then yes it's not a great fit but it's still passable if you're happy to give up transactions and the ability to force data consistency etc.

Where I work we have a production API serving thousands of websites that uses Mongo, don't sweat the details too much early on.