Just make the european window start in June and end before August?
Don't know. Sometimes the CL/EL final is in June. I'd keep it from 1. July to 31. August/1. September, but for all european leagues. That would be a first right step. Then after that maybe think about disallowing domestic transfers once the domestic leagues have started. That would mean you could still lose a player late in the window, but not to a direct league rival, which would give smaller clubs a little more safety in their plans, and you could still go out shopping internationally.