Considering you seem to dislike the whole concept of Bots, isn’t the Bot toggle in the Lemmy profile exactly what you want?

It auto ignores all posts and comments made by accounts marked as Bots. Sure, the owner of the account has to correctly mark it, but I feel that can easily be solved by reporting unmarked bots.

But I’m interested, if you would like to expand on it, how would you make a bot opt-in?
The only way I could think of is through PMs, but that would be cumbersome, and generally unfit for purpose, for all the involved parties.

No way. That’s illegal!