I recently switched to a used Minimal Phone (no longer being made sadly) and its been great for this. The best thing for me is to just make the phone completely unappealing to use, but still capable of essentials. A slow refresh rate eink screen helps a lot with that.

For the authors complaints about ads, I recommend paying for ad free versions of things you truly enjoy. Budgeting will mean you consume less and have fewer choices, but enjoy it more. And if something doesn't offer an ad free experience you can pay for, that's a great reason not to use it.