For ease of implementation, perhaps modify cup setup such that all cups end at the same time, rather than all beginning week one? Might make it more difficult on smaller nations’ teams finding scrimmages at the start of the season, but it could work if a transfer deadline for the tournament is deemed expedient.
(Please note: I am not a fan of the “deadline for cup” idea because the idea of setting that deadline requires a way to define when the important matches start [aka tournament equivalent of playoffs/of B3BBM bracketing], and different managers will define that differently. This comment merely serves as a possible “how” presentation for the sake of those curious for an answer.)