You're not alone. Here's how I solved it: Last year I really wanted a new smartphone just for the better camera. My existing phone from 2018 was still working fine, but the camera sucked.
So I bought a used, but only few months old, new smartphone.
And I never got it hooked up to the cell network (i.e. no SIM card). I now typically carry two phones on me. The old one is for texts/phones. The new one is for everything else. A clean separation. At times when I do groceries or something, I leave the SIM phone in the car so no one can contact me.
When the old phone finally dies, I'll just find the cheapest smartphone to replace it and maintain the separation.
For app notifications, I use the Buzzkill app to keep them down. For a long time I had it set up such that I would not get any notification for texts - other than a vibration. No sound. No flashing LED. And no notification in the task bar. If I wanted to know if I'd received a text, I'd have to open the app. I strongly encourage this set up.
Before I got a smartphone, I would turn my cell phone on only for emergencies and the occasional coordination (picking someone up - call him and let him know I'm downstairs). I told people they wouldn't be able to reach me on my cell phone, and to call my home phone (landline, and then VoIP) if they needed me.
Then I finally got a smartphone. I still have that home phone. But boy, I often tell people that my life is definitely worse because of that smartphone. I like the portable computing device, camera and GPS. Just not the phone part!
