Check out Aria the Masterpiece (originally released separately as Aqua and Aria). It’s mostly slice of life with some sci-fi and fantasy elements. The main character speaks very regular/standard Japanese, but of course not all of the characters do. I often recommend this manga for first time manga readers.
Book club: Aria the Masterpiece Home Thread (Beginner Book Club / Aria Book Club)
Buy on Amazon: https://www.amazon.co.jp/gp/product/4800005019