Yeah, the best way to describe various iM@S series is any adaptation should usually be seen as its own independent thing no matter it be the OG's, Dearly Stars, Cinderella Girls, Million Live, SideM, Shiny Colors, Gakuen, etc., and no matter the format.
Some are linked to one another, like the main anime adaptations are synced to one another for the most part, but that doesn't include the U149 adaptation which is pretty different from this manga, and things like the Cinderella Theater adaptation are more in sync with the actual game universe.
Some things, like the spin-off manga Asayake wa Koganeiro, do require some background knowledge, as that's based on the anime canon, and there's even some neat references to more obscure games like Stella Stage, but it's rather minor and it's pretty rare you need to actually watch the entire series for anything.
Ultimately, it's a rather large franchise that's plays fast and loose with itself, and as such continuity shouldn't be worried about too hard. For U149, every idol originated from the gacha game from 2011 and their backstories are mostly the same barring some key differences. Other than that, I wouldn't worry too much unless you really want to get more into things, in which there are a bunch of fan translations for the various events and stories floating about online.