Show a different view mode other than full by default when pressing the preview button

Seems sorta relevant/adjacent:

Entity Reference with LayoutIssues Use “preview” view-mode as default for display in edit form https://www.drupal.org/project/entity_reference_layout/issues/3045508

drupal 8 extend node form Went with the original poster on this one— not with any of the answers! https://drupal.stackexchange.com/questions/163607/alter-an-existing-form