Thanks for clarifying everything — because of the info you've given me, I can ask you a question that would usually come after some back-and-forth:

Would you use a web-based email client where, with one login, you can access the relevant ones? Not in an unified inbox, but have them as separates.

Say you add all of them but disable notifications on address #4 and set notification rules for #5.

As a consequence, you wouldn't need to forward #3 to #2.