How would Fortress Maximus be with a S/O with separation anxiety?
Fortress Maximus would try to keep stable contact with you. He made it clear to you that if you feel in any way that you need him, you call him to his private frequency and he will answer.
If you suffer separation anxiety so badly that you take it at yourself, like self-harming, he will have Rung, his most trusted friend to check on you each day and be with you, telling you that Fortress Maximus will return to you.
Otherwise, you have his number and no matter where he is, when he sees that it's you calling, he answers immediately.