I'm sorry, I'm not sure I fully understand. You can propose in Step 3 but the actual wedding will only ever happen with their Step 4 versions. But you don't have to do things during the transitions. If you play the DLC Moments, you can propose to Cove in Step 3 with an actual scene. If you don't have the DLC or just want to wait, you can propose to him in Step 4 itself or Cove can propose to you in Step 4. There's tutorials on how to get those scenes in an FAQ-
https://gb-patch.tumblr.com/faq
It will be the same kind of thing for OL2.