How to fix the SWF Issue
Just don't display survivor names in game!
That way they can't look each other up on Steam and chat up. You've already blocked out the killer builds for SWF groups, this is the next step. (and actually if you do this you can unblock killer builds because casual SWF survivors can't chat with each other anymore)
You'll still get actual friends who play together with chat, but those kinds of people are not as manifold nor as problematic as the ones who just buddy up in the chat to get an SWF chat advantage over killers.
The ones killers complain about after all are the SWF fly by night players who make SWF so common, right? If it's restricted to just actual friends who really play together this won't be such a big issue.
Or at least, it should no longer be used as a scapegoat excuse by killers for why they lose.
Also, killer shouldn't see the survivor name either. That way we can avoid griefing people you hate, or dodging certain players. Just call each other by their Survivor name -- all Claudettes will be called Claudette. If there's more than one one is C1 the other is C2.
Also, remove the lobby chat. Stop letting Survivors prep and tell each other their name or Discord channel. And also don't let killers see survivors, so Killer's can't prep either. Fair is fair.