you migrate into a database when you find you need an index, or a join. This migration is easy to go from a flat file into a database tbh. And you can do this

you migrate into a database when you find you need an index, or a join. This migration is easy to go from a flat file into a database tbh. And you can do this migration in pieces - copy the flat file and perform the ETL as a separate thing, all while your existing app runs fine unchanged. You can then implement the needed joins or indexing on this copy in the DB, and then schedule some sort of job to re-import the DB from the flat file. Sure it's not real time, but it's enough to implement most features, with little effort from migrating the read/write code in your app from flat file to the DB. And nothing stops you from migrating the read/write api from flat files into the DB, after you find that it is really needed.