Reg: Migrate from Firestore or keep MongoDB for database and Firestore for other services

We have our flutter based APP with back end created on Firestore(NoSQL). Its a reminder / ToDo app with simple database and calculations.
How easy is it to move the back end to MongoDB from Firestore and what preparations should we take care in the back and front end of the APP before moving? Is it possible to sync the database to MongoDB frequently, even if not moving immediately, while the production still connected to Firestore and change the back end to MongoDB when the app grows, with less downtime and just make the changes to the Frontend/client side? Or keep the database as MongoDB while using Firestore for the rest of the services which are missing in MongoDB? Please explain in detail and share any links that would help if possible.