I didn't see Spawn Astarion's reaction as a show of insecurity about your relationship, at least not completely, more as an insecurity about how to be in an equal relationship. For most of the game he has been trying to manipulate you into doing what he wants, now you are partners he doesn't want to do that any longer and takes the decision-making to the other extreme and becomes a bit too careful. I don't think an "I love you" adequately fixes the lack of actually doing/deciding things together or makes up for not being able to tell him how highly you think of him - in case you do, of course. I'd also wish the affirmation in the couple scene at the end was more enthusiastic - it would have been such a great place to fire that "There's nothing I want more" back at him.
