You say upper intermediate but that you’ve never read manga before. Do you have a lot of reading experience from newspapers or other sources?
Anyway, the first (and currently only) series I read completely was 放浪息子 (discussion thread) and I quite enjoyed it. Mostly has no furigana, so if you don’t know many kanji beyond your WaniKani level would indicate it might not be the best choice to start with. The language overall isn’t too difficult though. Definitely worth reading at some point if you like good character-driven dramas.
A good manga to start with is Aria the Masterpiece. It has furigana for everything if that helps. While it has sci-fi elements, it’s more slice of life iyashikei than anything else.
Other than that, perhaps join one of the current or upcoming book clubs so you can ask for help if needed. By reading with others you can also try material at a bit of a higher level since there’s a support system of people helping each other out.