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I see you've watched the anime so that's a start. It looks like you're currently playing Platinum Stars, the newest game. You also mentioned you played One for All and trying to finish Idolm@ster 2. After you finish all these games, the only game I would recommend is Idolm@ster 1 (xbox 360). But, it maybe difficult because of region lock, so you can just watch online videos.
Seems like you've been a im@s fan for a long time. You probably already done this, but keep exploring other im@s stuff such as the music albums, mangas, dramas, Movie/OVA, etc. We also have a wiki here, which has a wealth of information about the franchise.