This is a very, very painful issue for many modern shooters who have moved to cross-platform play. Cross-platform is the reason balancing becomes extremely difficult, and the opportunities for cheaters increase.
I see only one solution tha split accounts into verified ones (authenticated by ID or phone number) with high trust so losing such an account would be painful, and have those trusted accounts mostly play with each other. Then have a sandbox for new and unverified accounts so they can grind hours and build up reputation