Since this forum is far newer than the other, there isn't as much in terms of guidance and suggestions, and searching got me veritably nowhere. I've done RTK1, and I'm about 300 words into Japanese Core2k, which I'm finding easy (mainly because I know a lot of the words already) and fun. So where would this leave me if I wanted to try Chinese? I take it I shouldn't bother with RTH. So, (bearing in mind I'm a heavy anki user) what's the most popular next step?

Edit: I have tried the zhongwenredgreenblue deck, but it's rather confusing jumping straight into sentences with zero vocab. Or is that kind of shock-immersion actually a good idea..?

There's a Chinese equivalent for the Japanese Core deck:
Anki - Smart.fm - Chinese Sentences and Vocab

It has vocab and sentences in separate decks. (It would certainly be possible to merge them like in the Core deck, with the word and its respective example sentence in the same card, but no one has done it yet.)

I haven't tried many methods yet, but what I like about the zhongwen resource is that it provides a lot of context for the keywords or key grammar points of a lesson. I found that hearing and speaking Chinese is a lot harder than Japanese, because of the (to me new) tones and (in my language) homophones. So I focus on listening (vocabulary) at first, even though my long term goal is to read literature. Maybe you experienced it as shock-immersion because you're used to first learning vocab (out of context) and then doing sentences.

When I started out with Chinese, I used a HSK basic vocabulary deck. It's in the shared decks in Anki I think, and it includes generated audio (I mean like the pronunciation thing Google Translate has). I think generated audio is actually not that bad with Chinese in the beginning stages, because they pronounce quite clearly, and the syllables are mostly separated in speech anyway (well in slow speech at least, but when you are a beginner fast speech is too fast anyway). Be aware though that some of the sounds are wrong. If I remember correctly, neutral tones are always pronounced as tone 4 in the HSK deck. If you are not sure, perhaps another deck would be advisable. I did Zhongwen and am doing the Core Sentences deck right now. Both decks are very good, but perhaps jumping right into sentences might not be the way to go. For me, I do sentences to get used to the grammar (when text is on the front of the card) or to practice listening (sound on the front). This means that ideally you should know all or most of the vocab in a sentence. Learning vocab through sentences is not the most effective way in my opinion. But this is my personal preference, do what suits you best.

On a side note, I'm doubting if starting Japanese and Chinese at (roughly) the same time is a good idea. If your Japanese is not very solid yet and you haven't seen the readings of most of the Kanji, you could be prone to mix up the readings. Of course this can always be a problem, but once Japanese has really sunken in your system, the chances of mixing up are smaller. So instead of doing Chinese, I would just double your speed in Japanese for the time being. When you have something like 5000 words under your belt (which in all honesty is still early), you could start with Chinese. And you are right, in my opinion there is no need to do RTH once you have done RTK and have a solid Japanese foundation.

Of course, anything is possible, so if you really want, just go for it.