That’s better than me! I passed HSK 5 when I was 15 or 16, I think. I didn’t dare to take HSK 5 at the time because my Chinese wasn’t good enough. I’m not sure I would pass HSK 6 now though, since there’s that ‘text summary from memory’ section, but I’ve improved a lot since then. (I’m ethnically Chinese and started as a toddler too, but I grew up in Singapore, where English is usually the everyday language. Chinese is my second language, just as it is for you!) Anyhow…
I don’t use WaniKani at all because I’ve been telling myself I’m just doing exactly the same thing as I did for Chinese, so I don’t need a special system to help me. I’m just here to use the forums. However, there are at least two or three Chinese speakers here who seem quite invested in using WaniKani, so they clearly find it useful, at the very least for handling the differences between Chinese and Japanese. You’ve already reached Level 3, so I guess you already have an idea of whether or not WaniKani is helping you.
Radicals are really useful, in my opinion, but if you can already identify radicals in Chinese, along with their names (e.g. 木字旁, 宝盖头, 病字框), then there probably isn’t much WaniKani can teach you about breaking characters down. As @StarPT said, WaniKani radicals are made to help people remember kanji, and the names used are often nothing like the official names that Chinese or Japanese scholars would use. For example, 丿is called the ‘slide’. That doesn’t exist in academic parlance, and might not even be a ‘real’ radical, but it’s easier for people to remember. The way WaniKani works is that it teaches you its system of radicals first, and then builds mnemonics using those radicals so you can remember new kanji. Whether or not you should learn WaniKani radicals depends on whether or not you’re going to use WaniKani mnemonics. If you’re going to be using your Chinese knowledge to spot classic radicals that convey meaning (e.g. 火、氵、冫、扌and the like) rather than use WaniKani mnemonics, then no, maybe you shouldn’t bother with WaniKani radicals.
How then should you use WaniKani?
First, you’ll have to decide if you want to use WaniKani-style mnemonics. If you’re going to do that, then all you need to do is follow the path that’s been laid out for you and pick up the radical names given by WaniKani, then apply them in mnemonics. WaniKani provides mnemonics for both readings and meanings – they’re usually separate – and they all rely on WaniKani memory devices like ‘cock’ for the sound かく (remember to pronounce the English words with an American accent) or ‘Mrs Chou’ for the sound ちょう, or Kouichi (one of the founders of Tofugu and WaniKani) for, well… こう, or ‘rock’ for sounds like ら… You get the idea. How much of the system of mnemonics you can use depends on whether or not you already know the system. One caveat: since you passed the HSK, I’m presuming you, like me, grew up with Simplified Chinese characters. Japanese kanji are generally much closer to Traditional Chinese characters, so WaniKani might be able to help you with that. Honestly though, it’s much faster to just compare traditional and simplified forms and learn a bit of 草书（草書 in Traditional Chinese and Japanese）in order to understand how the simplification happened. That’s my opinion anyhow, but then again, I have some calligraphy experience, so looking up these sorts of simplifications is natural for me.
Next, you’ll need to decide what you want to learn from WK, since you probably already know most of the kanji here:
If you thinking of learning meanings, WK probably won’t be that useful. Most kanji mean almost exactly the same thing in Chinese and in Japanese. The main differences are how kanji are put together (e.g. 投与 means ‘a dose of medicine’ or ‘the act of giving by throwing’, which are meanings you can guess from Chinese. However, the phrase 投与 doesn’t exist in Chinese.) and the specific nuances they have (e.g. 適当 in Chinese always means ‘appropriate’, occasionally with a euphemistic nuance of doing something nasty to ‘appropriately’ dispose of a problem, whereas in Japanese, 適当 can also mean ‘to be characterised by doing the bare minimum in order to deal with something’ i.e. to be irresponsible and careless). WK might, however, teach you meanings you don’t know, since some meanings of kanji in Japanese are closer to how they were used in Classical Chinese, and allow you to drill vocabulary that doesn’t exist in modern Chinese. If you’re set on using WK to learn, I’d suggest that you pay careful attention to meanings that seem very different from what you’re used to in Chinese. On the other hand, for WK definitions that simply use a different word from what you’re used to, I suggest you make heavy use of the ability to add user synonyms. That way, you don’t waste time trying to remember the specific word WK uses. After all, as a Chinese speaker, I’m pretty sure you know exactly how many meanings a single kanji can have. Reducing them to a single word, especially if that word is narrow, can be troublesome and unhelpful. Plus, some meaning choices on WK’s part are really problematic: 失 is taught as ‘fault’, whereas almost every meaning it has in Japanese and Chinese is related to ‘loss’. Use user synonyms to make your life easier and to save you time when learning meanings.
If, on the other hand, you’re thinking of using WK for learning readings, then I think you might find it helpful. You’ll have to use the WK mnemonic system or create mnemonics of your own, but since Japanese often uses very different readings from what we have in Chinese, you might find it helpful to have a system like WK that will help you drill these things. My suggestion is to look out for similarities between on’yomi (Sino-Japanese readings) and readings in Chinese. As a Chinese speaker, you should rapidly notice patterns that will help you realise that as Chinese speakers, we can often guess on’yomi based on readings in Mandarin, and often without any help. If you or your parents speak any Chinese dialects other than Mandarin, you’ll probably find that knowledge very helpful, because Japanese on’yomi are much closer to non-Mandarin dialect readings. I analyse some of the common patterns here: https://community.wanikani.com/t/anyone-else-making-the-jump-from-chinese/47441/3?u=jonapedia
Finally, I guess I should just say that I don’t think you’ll see many unfamiliar kanji until you reach about Level 50 of the programme, where you’ll start seeing kanji that are common in names, but not often used otherwise. I think it’s good to be aware of that, though I don’t know what your expectations of WK are.
I’d like to suggest that you also study Japanese grammar and vocabulary on the side, and that you frequently expose yourself to Japanese through media that you enjoy, like anime, music or dramas. WaniKani can only do so much on its own, regardless of how well built it is. With that, I wish you all the best, and I hope that someone else who has actually used WaniKani as a Chinese speaker will turn up to offer you more specific, experience-based advice.