My apps are mostly one-purpose. They have a very simple UI, because i believe, that the main thing is the functionality. I use the Tasker framework which allows a deep integration with the Android system.
When you unlock your phone, you decide for how long you want to use it. After that time, you get a reminder with only one option - lock the phone.
This app should help with limiting screen time.
Morninger
In the morning, your phone asks you if you just woke up. If you did, the phone will remind you to close the phone and won't give you any other option.
This app should help with making a better morning routine.
MCBM (Minecraft background music)
An app that plays Minecraft music in the background. You can customize it a lot, you can even determine exact times between which the music can't be played.
This app should improve the atmosphere of normal moments.