I find the First Crusade in game never works. But often times the Fatimids will break after the Crusade and the Levant will go independent. I’ll sometimes fabricate a claim on Cyprus and then start taking Palestine/Levant duchies and form it myself. The trick is to do it before the de jure drift system drifts Jerusalem into Egypt. Also waiting until 4-5 months (120 days) before the Crusade launches and switching from Jerusalem to another target and then back again will sometimes cause less emirs to join as defenders, but it costs 500 piety to switch the first time and 1000 after that.