Chimera Squad flows differently than XCOM and players get used to sending the same team day after day, fatigue scarring ends up seeming like punishment to the player once it would start happening. How about switching the penalty for going on missions with a bonus for being "Well Rested" instead? This way it can be implemented from the start and still encourages changing squad mates. It could be a buff to XP gains, this way there would also be no need to make idle members earn XP for sitting around at HQ