hi Ogga welcome here.
For the info you provided above, the first thing I’ll do will be to treat your 2.5d avatars as it was 3D, so i.e. you just switch what is now the smiley model in one of these steps with your flat avatar and keep parenting with 3D colliders. As simple as it sounds. So, try by yourself to modify step9 smiley with one of your characters and see if you are able to make it work, otherwise come back here with your code and we’ll see together what is wrong.