What apps or tips do you recommend to "dumb down" your smartphone?

Uninstall apps that have a web app, particularly if you use it rarely.

Some phones have an option to make the display black and white either all the time or on a schedule (at night)

Disable notifications for apps that aren't important. Set less urgent app notifications to "silent" so they don't pop up.

Disable email notifications entirely.

Set your phone to silent when you're not expecting important notifications.

Prevent the screen from turning on when notifications appear.

Go through every single setting on your phone, carefully consider each one, and choose the less intrusive / distracting option.

Relatedly, I've also written up what I've done to make Android a bit more private.

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.

I limit social media to certain times and places. Kills doomscrolling!

Less is more. The only App that actually makes sense to add to your phone is maybe a minimalist launcher App. there are several options on F-Droid of them. Otherwise, to "dumb down" you just need to uninstall everything useless. Even prefer Webapps over normal Apps if services have them. This way its just a little more inconvenient to use your phone to much if you always need to go in the browser and search it.

You want less phone. Not more. So I really don't see a need for anything else then a different launcher.

Use Progressive Web Apps instead of downloading apps. Limits information leaked about you. Also has the added benefit of making a terrible experience for apps like instagram. Hard to doom scroll when the PWA constantly messes up and you have to reload.

Delete social media.

Buy a pocket sized e-reader. I recommend options from Bigme and XTEink.

Learn to reach for books instead of apps.

Always run the debloater script on a new phone and check once in a while whether upgrades add new crap, like the AI Service Engine they secretly installed recently.

Also install Tracker Control.

ScreenZen, with specific times ranges allowed to use given apps.

If your phone supports Lineage, the default installation comes with only the apps you would typically expect on a dumbphone (except the web browser). You can just never install any stores, never download apks from the web, and chill with it

(iPhone) Depending on the framing of dumbing down, an alternative camera app could be suggested. I use Halide to get rid of all the photo processing Apple does by default (which ruins night photos imo).

When you put away your phone, place it just out of reach.