Yeah if you're going to change prepared invasions, making it more inconvenient for the player while keeping requirements the same for AI is for the best.
In that vein a Holy Fury style Crusade really is the answer. In that the lands you win are distributed among randomly generated courtiers ("participants.") Give the courtiers a few male dynasty members to prevent it from dying off instantly from one errant murder. The player would then need to plot to revoke and other CK2 vassal management shenanigans to get the lands they really want from the invasion, as opposed to just outright blobbing. In this way dynasties such as the de Hautevilles can emerge.
Increasing restrictions to fire when the current Prepared Invasion format isn't even abused by AI is not a good method of changing this.