A perspective from an European here with nativeness and alike in several latin-related languages and long lasting interest in Japanese language and related culture since the 2000s.

Certainly for written Japanese it will help you your Chinese knowledge (after a learning curve of false-friend associations), I heard that many technical/modern words have been imported in Chinese also from Japanese adaptations (only the characters implied, not the sounds, as is common in indoeuropean language imports), as a return kind voyage, since Japanese writing was first imported from old Chinese and then evolved in today’s system (kanjis and two silabaries). Also many English words have been imported into Japanese, but highly phonetically distorted in the adaptation. Foreign words are easy to spot in written text, and I often chuckle when I understand the word by realising about the original one after backtracing the intended pronunciation.

As a consequence of Chinese influence in the writing system, most of the kanjis have two pronunciations, one(or-more-alike) of Japanese origin and another(or-more-alike) of Chinese origin, which in many cases will resemble to current Chinese ones, but I have heard that phonetic changes will throw away potential direct understanding (also rules about which pronunciation is used when in Japanese are not rock solid or straightforward always) specially since grammar is notably different also. I found that proficiency in two similar related languages (e.g. between roman-latin languages, between germanic languages, etc) develop certain ability in spontaneous word recognition across phonetic variations, but I found this in indoerupean languages with “long” words with “long” roots (not one “syllable” per “word”), not sure how much would work between Mandarin and Cantones and a phonetic adaptation from old Chinese into Japanese, which would be just a part of it.

I am far from fluent in Japanese, but the most basic interactions, grammar recognition, etc and the learned nuances add a wonderful experience to OVS watching (love for those sub volunteers that explain the cultural context of many situations), and since most of my consume is Japanese culturaly rooted (e.g. not sci-fi, western fantasy, etc) I am not interested in dubbed material at all. I think fluency requires a serious investment, even for Chinese background, user abilities and environment may vary this a lot also, so the gain must be worth it: for careless plain consumption of works not rooted in Japanese culture I doubt is worth it, for the rest I find worth the effort to read subs most of the time and appreciate recognise the nuances hard/impossible to translate.

I had zero regrets of all what I invested in Japanese understanding up today, even if is not enough for general understanding, but I also find such cultural travel worthy on each step. I am attempting something alike with Chinese nowadays, let’s see how far I arrive…

Good luck!