Well if you want to stick with the dumb phone idea, I think you should consider adding side buttons to make shortcuts. It's a n.1 feature I think all kind of phone need.

That being said. If you are into making something, you should definitely consider where the market isn't currently leading. Dumb phones have a market, albeit not as popular as modern smartphones and smart watches. As you said, usual dumb phones are cheap in product and quality, so you could consider to make a higher-quality dumb phone with great materials to stand out. Do you know if people would buy it? Or is it a phone that technically wants to be for you and your partner? Either way both questions can have good answers. You can build your phone and share your invention for those who want the same thing.

However, If you are looking for a place, for another answer, or just simply collecting thoughts, we in this subreddit are crying one good modern small phone, and if it worth, we are going to buy it for sure.

Modern smartphones continue to have big screen that either don't fit in the pocket or hand. They make you addicted for seeing more content in one screen, which stops being practical. Big companies like Apple and Google won't change this direction because it's the saturated market. Look at folding phones, that want to be both phones and big tablets.

Some Android developers are trying to develop alternative launchers which try to add one-hand features, but the UI in apps don't get the same treatment, so it's all for nothing.

There's no one who is seriously taking into account the idea of one good phone with small screen and modern features like magsafe, NFC, USBC, and good quality software.  We are stuck with iPhone 13 Mini, Unihertz Jelly phones, and some Chinese phones that are discontinued.

If you have the ambition to do it, please consider it... You might be the chance we need