You can start with the 2018 anime (12 eps) then follow up with the movie (2021). The Rondo Rondo Rondo movie (different from the 2021 movie) is a retelling of the anime with some additional scenes so I would say you can check it out if you want but not necessary imo. If you really like the show after all that you can check the stage plays at least in their manga form (on dynasty) and overture (manga, also on dynasty), then you can play the visual novel available on steam and on nintendo switch
This is all additional info but the anime is the way to go