You might be asking the wrong question. The first answer you need to know is why a dumb phone is better than just under-using a smartphone?

I can think of a few answers to that, btw, but before you try to start knocking down roadblocks of what use cases dumb phones cannot cover, figure out whether your target audience's desire for a dumb phone is even about the features, or whether it is cost-based, or psychology based (to avoid addiction, etc.), or is just trying to avoid owning devices that are run by big tech companies.

Personally, I barely use my phone. I forward calls to my VOIP line, I handle texts and other messaging from my PC. I don't really use apps other than the occasion bird or plant identification, some maps if I am travelling to a new location, and one notable time when I locked my keys in my car and used an app to unlock the doors. All of those scenarios could be handled without a smart phone, so it is certainly possible to drop the phone. But in that case, I would not get a phone at all.

So I feel you are looking at a niche, and one that is more about lifestyle choice, not feature sets.

“The smartphone only comes out for banking or government apps.”

You answered your own question.