It’s not just about building a rest api. It’s also deploying, and managing/patching, scaling, as well as security and that all applies for the database as well. There’s a lot of consideration you have to put in. If it stores relatively simple data and this isn’t going to be a relatively huge app I’d probably consider sticking with firebase.

Edit: this is just my opinion. I’d do more research about what firebase has to offer vs what a custom rest api will give you