I just rewatched Drive My Car. The director has a new movie that's quite different, but if you want to watch a deep exploration of grief and regret, that's your movie.
Haven't watched Drive My Car yet, but just from the trailer it looks like it's exploring loss and grief rather than the most poignant type of regret of losing something due to own action (or inaction). I.e. having agency and being directly responsible for loss.
Now I wish I knew Uncle Vanya better.