You could just use a computer without internet. Get something without a network card. But your use-case is quite bad as you still need access to the internet and you even want Facebook, Reddit & YouTube, which totally will send you into your doomscrolling dead spiral again.

So some hints:

• you can download YouTube videos and view them offline
• there are lots of ways to disable the internet or slow it down. Nothing will work when you’re the admin as you can and will disable them.
• if you get distracted doing your work, even black & white displays will not work. People totally didn’t do their work in the past and watched a lot of black&white television or even read some books