Ah, well it wasn't even clear to me that you were talking specifically about lemmy. Other software already has some of the features you want such as federating only with a chosen few.
It seems the main thing you actually propose is a new system for categorizing potentially undesirable content. I would suggest looking at what has already been tried in that area. There's more to it than you'd suspect. How to choose the categories will never be uncontroversial, and coming up with a standard that any large fraction of the fediverse might accept would take some actual work.