These are good questions. A small board does not allow for endless doomscrolling and does nothing for the addicted. People who will try to replace a big board with a small one will become discontent. Slow = "the dopamine comes slower than before".
Therefore, I can see why one will try to quit cold turkey. It's a reasonable thing to try when all else fails.

Usenet groups. Yes the absurdly large group tree is a burden and does no good. Some slashdot people had similar idea as you did and picked one already existing group and posted or crossposted all their tech-related messages into it, as an attempt to revive usenet. So it has been tried at least once in relatively recent times (2015? 2018?)

>>5 Found it. Pony "a messenger for mindful correspondence".