There’s a lot more to the genre, and the isometric shooters you’re talking about are technically not shmups, rather they’re twin stick shooters. A true Shmup is the bullet hells you mentioned, but bler hell is actually a subgenre, there are many types of shmups that control the same way a bullet hell does, but minus the hell of bullets. Check out Shmup Junkie on YouTube and join his discord to learn more than you ever imagined about this amazing genre!