IANAL , but theoretically a judge could decide that part of the AGPL is “unreasonable” and a case could go to trail which could be expensive.

Also you are still running the lemmy.ml instance which could theoretically lead to a lawsuit. its why lemmy.world created the fedihosting foundation which reportedly functions as a limited liability company.

Also a cooperative (what you want to set up), where some of the profit in the form of a salary go to the “owners” which are the workers as far as i can tell can’t be a non profit

That is true, but not so easy to do. To make donation pages like those in your link would require setting up some kind of nonprofit and directly handling credit card payments with some payment processor

Shouldn’t it be a good idea anyway to set up a limited liability company at some point anyway? in case you will get sued.