I have been studying for roughly 9 months. I've finished all the N4 vocabulary terms and I don't need to study kanji because I'm fluent in Mandarin. I've also done a bit of grammar work, but I find that even if I understand all the components of the sentence, I still don't understand the overall meaning combining all the terms together
I don't need to study kanji because I'm fluent in Mandarin.
By meaning, usually they're the same, but there's distinctive usage in Japanese that sound is going to be different. Over time it would be easy to forget and muddle it if you relied on Mandarin.
At N4 grammar, though I'm not at that level really, I don't think it to be different to N5. Same beyond that. Not that you say you have trouble with learning the grammar concepts, it seems you really have trouble comprehending them in practice. And I would guess you're trying to rely on memory alone. So this is your weak spot.
It wouldn't hurt to review the grammar and grammar lessons while you do your reviews. You can also do that for reading. I'm of the habit of making my own notes to guide myself into thinking in Japanese as well. Let that be your cheat sheet for a while until you're used to it.
Knowing Mandarin helps a ton with kanji, but you still need to learn all of the different readings. There are way too many differences and false friends. There's also 連濁 and 当て字, among other things. 丈夫大家手紙 ← These look the same, but aren't even close in both reading and meaning.
That aside, nine months isn't a lot... what you're experiencing is very normal.
You also need a lot of real exposure to the language. Platforms like Renshuu are just one piece of the puzzle.
Immersion is really the key here, and it's a highly debated thing.
The most typical and recommended form of immersion is i+1, which is to say only slightly harder than your current level.
There are lots of movies, tv shows, books and comics aimed at children, and these will be your best bet. The absolute easiest to start with is (in my opinion) the comics and tv shows. They're shorter, have pictures for context clues, and are overall fun to use.
I don't really recommend music for immersion because it breaks a lot of grammar rules (because it's music).
Assuming that you have already mastered particles (If not, that is the first step to understanding sentences), the next step is to practice breaking down sentences a lot. I'd recommend checking out the "Japanese from Zero" videos series, where they regularly do these types of exercises (especially from Book 4 and on). Once you get used to the process, it becomes natural.
You'll need to identify specific sentences where you know the components, but can't figure out the meaning, if you want more specific advice.
I guess so. For exposure, I don't really do much other than listen to Jpop which I don't understand 
