Which is the better way to white-label a flutter app

I have one project that I have to sell to another clients, so I wanna found a way to unify the code to, when I release some updates, I have to manipulate only one code (and, of course, keeping the specificities from each one)

I found an article HERE which the guy creates a new folder named 'config' and set some variables there to be used in the parent project. I tried this but find out that would be very tough to do because the first app was developed specifically by one client, and with it I would need so much time to make all the aspects dynamic... Another problem is firebase, in first app I used firebase but in the second i won't. How to make it possible?

And in this article they say about 'flavours' that can be used to do something similar.

Someone knows about this approaches or there is another to reach my goal? With flavours I will have less re-factor than with config?

I appreciate any help