There is nothing stopping you from using only these three options now. Your problems are in your head.

I keep looking for a smartphone in a feature phone's body.

I don't care about a big screen or zippy processor. But there's all those handy little apps on Android that I'd have no replacement for.

Google Authenticator is the only app I actually need, it's required to authenticate for a whole bunch of services at work. My employer paid my iPhone, and future upgrades, so it's their call, but I'd be just as happy with something like Nokia 104. Assuming that I could get an Google Authenticator app for it, my employer could save $1000+ every 18 to 24 month.

There is always Windows Phone ducks

If your feature phone has FB messenger then it (probably) isn't more private than a regular smart phone.