i've probably studied 5 hours of mandarin by now... it's helping me sooo much with japanese! the biggest help is kanji meanings, and the meanings of the parts inside each kanji, but it's actually also helping with pronunciation. people say the grammar isn't the same between the two but actually they have some similarities due to stuff like japanese trying to literally copy chinese grammar hundreds of years ago, or due to almost all high-level japanese "grammar" actually just being vocabulary words (with kanji taken from chinese), so it even helps there.

so yeah, even if nothing else but for the sake of my japanese, i'm definitely sold on the idea of to "just keep learning chinese until i'm fluent".