very late reply, I saw your post only now:

I’m in a similar situation to you, I also learned Chinese before Japanese. I went to Chinese classes once every two weeks and didn’t do much independent study, so it wasn’t ever very serious, and I would say that even after 7ish years I couldn’t really converse much, also due to the fact that I hadn’t had any opportunities to speak to native speakers of Chinese. However, one thing I really focused on in my study was writing, and by the end I recognized 1000+ hanzi. Then I kinda abandoned it for a few years, and only two years ago I started with Japanese.

So, with this experience, I would say it helps A LOT in two aspects of the language: kanji recognition and vocabulary.

Firstly, kanji. The vast majority of kanji share meaning with their corresponding hanzi. There are a few cases where the meanings diverged, but these aren’t that common. Also, in most cases they look the same as traditional chinese characters. Japanese kanji sometimes have a 旧字体 (kyuujitai) and a 新字体 (shinjitai) form. Kyuujitai and traditional chinese are the same in 99% cases. Shinjitai can be thought as the japanese equivalent of simplified characters. In case of Japanese, the writing reform was much less extreme, only a small fraction of the characters changed, but most of the changed ones happen to be frequently used. Sometimes the shinjitai characters evolved in the same direction as their chinese simplified equivalents, e.g. 点, 国, 広. Sometimes they evolved in a different direction: 氷, 囲, 缶.

Japanese kanji fundamentally have two types of readings: kunyomi (native japanese reading) and onyomi (chinese reading). Kunyomi readings are native japanese words that share the meaning with the character. Example: uma is Japanese for horse, and so 馬 has kunyomi “uma”.

Onyomi are readings that resemble the chinese reading at the time words containing that character were borrowed. Example: 馬 ba like in the word 馬力 bariki - horsepower. Most of it happened when Middle Chinese was spoken in China, so onyomi are most similar to Middle Chinese readings. They were adapted to Old Japanese phonology and since then both Chinese and Japanese phonology has evolved a lot so modern Mandarin readings can be quite distant from modern Japanese onyomi readings. Mandarin specifically has lost the -k and -t codas (corresponding to Japanese -ku/-ki and -tsu/-chi), but many other Chinese languages haven’t, for example Cantonese, so you might find onyomi to sound closer to modern Cantonese than Mandarin.

One interesting thing I found while investigating the relationship between onyomi and Mandarin readings is that the characters whose Japanese readings end in ku/ki/tsu/chi are almost always read with the 2nd (rising) or the 4th (falling) tone in Mandarin. Once you learn onyomi for many characters whose Mandarin reading you know already, you’ll notice patterns and you’ll be able to predict the onyomi. It is also somewhat possible to predict the Mandarin reading given an onyomi, however, it’s much harder since there are much more possible onset-rime combinations in Mandarin.

Some characters have multiple onyomi readings. This is often due to the fact that words using these readings were borrowed into Japanese at different times, for example, there are some buddhist terms that have made it into Japanese much earlier than most borrowings from Chinese, so they use older readings.

Finally, vocab: a significant portion of Japanese vocabulary are words borrowed from Chinese. They are usually kanji compounds containing two or more kanjis. They are read with onyomi, so they sound somewhat similar to the modern Chinese pronunciation. Example: 銀行 ginkou is cognate to Mandarin yínháng, they have the same meaning and sound similarly.

I realize I diverged from the topic a few times so let me summarise: If you learn Japanese after learning Chinese, for every kanji you learn, you will already know its meaning and will be able to predict its onyomi, leaving only kunyomi to memorise. Additionally, you will already know the meaning of most of the borrowings from Chinese, which make up a significant portion of Japanese vocab.

I don’t know a single word in Chinese but I’m almost sure it will be easier than starting “from scratch”.
Kanji are Chinese characters adapted to the Japanese language. Knowing Chinese meaning should make it easier to remember those as from what I heard 80% has the same meaning in both languages. Kanji also have two or more readings, onyomi and kunyomi where the former is based on original Chinese pronunciation.
The writing should also be easier because if you can write Chinese that means that you will only have to learn hiragana and katakana.
But why just not give it a try? You don’t have to commit to it right away. Even learning a little should bring you a new context and look to another culture. Those hours definitely won’t be wasted.

They are way more Chinese words (at least 4X) than kanji. Of course, it will help you to read kanji since kanji are from Chinese words. It is fascinating to see how closely related they are.

You can try to learn a language well or learn two languages not so well.