USA centric info below:
A lot of the flip/dumb phones that are newer/would support things like Voice Over LTE (VoLTE) are either KaiOS or Nokia's Series 30 (or a variant of that).
KaiOS isnt that "dumb" and has got some funds/interest from Google, and has apps like WhatsApp - https://www.kaiostech.com
The Nokia devices that don't have KaiOS are like the 225, they come with Facebook preinstalled though I am not sure if it's an app or a web wrapper/link - https://www.nokia.com/phones/en_us/nokia-225-4g
If you're looking for pure tracker protection things like this could work (since you're not doing anything but calls and maybe SMS) but as you point out there is a productivity trade off - how far you are willing to go depends on your threat model and what you can live with.
GrapheneOS is more secure but is not always as private, there have been multiple discussions over the difference and the GrapheneOS website still calls some of these out if I recall. It is better in the fact that there is no Google, but the point isnt to be as private as possible but rather be secure (which comes with some privacy by default).
I don't have a good answer for you, but hope some that info helps.
Edit: forgot to add, but you could try out Linux devices like the PinePhone or Librem 5 - if you like to tinker and get involved they're good options, but some people don't find then usable depending on what you've come to expect from a mobile phone (the software is still being developed, it's moving fast but will have hiccups compared to other devices). Worth a look though if any of that sounds interesting!