This is an extreme example if you really want something like an old, 'dumb' phone from the past while still using modern hardware:

• Remove any non-essential apps to making a call or text. I would only keep F-Droid to keep your launcher up to date (unless there's a built-in updater)
• Replace your current launcher with something more minimal like Minimo Launcher
• Optional: Install a game (Just one!) like Breakout 71 for something to do on your break (Like how old phones had a game to play)
• Disable all notifications not related to phone or text
• Disable Wi-Fi, Bluetooth, All Location Services, anything that isn't essential to making a call or text.

EDIT: forgot to add:

• Only re-enable Wi-Fi for updating your launcher and/or game. Disable when you're done.

Congratulations, you've successfully 'dumb downed' your smartphone.