Honestly a big part of this is that the "dumb" phones are just as, if not more, expensive than a used or low end smartphone.
My daughter did use a dumb phone for a while, but then they started shutting down 2G and some 3G masts in the area and it was causing problems. There are some newer dumb phones that can use 4G, but given the prices and half of them still using micro usb for some damn reason (I'm a big fan of just carrying around one charger) it made more sense for me to give her an old smartphone and lock it down a little.
Is there some kind of parental software that can lock down phones like what happens on corporate phones? Like where you can control which apps are installed etc? That sounds like the best approach: just give them a phone with WhatsApp, phone and SMS. Any refurbished phone from the last 10 years would work for that (lack of Android security updates might be an issue though)
Family Link works okay for Android... It is google first party. Wish I could find something that worked for multiple platforms in the same app, but also that would probably be its own nightmare.
Off topic, but I bought a handful of magnetic cables and a few different ends to match our devices, and now one cable works for charging everything.
It has the downside of not letting you connect the device to a computer, but because they're magnetic, they're breakaway cables. My kid has walked away with their tablet after forgetting to unplug it, and nothing bad happens 👍
Careful with those, almost started a house fire once because the connection wasn't as tight as it needed to be. With it being magnetic and carrying power you run into the risk of metal debris magnetising itself on it and completing a circuit