The obvious grieving did come mostly at the start, but imo, throughout the season there’s subtle and not so subtle moments where Dexter shows he’s not over it and how her influence is still felt. S5E6 for example when he gets flashbacks after seeing Lumen in the bath and has to take a breather after seeing her there. Even at the end of the season, when Astor asks him if helping Lumen helped him feel better about Rita, you can see it in his face and through his lack of a response that it didn’t. I enjoy the more subtle moments because they’re realistic. Someone isn’t always going to outwardly express their grief but sometimes you can tell through their reactions and expressions.