Anecdotally speaking, my spouse is from China and she thinks it was easier for her to learn Japanese than English despite learning English from a young age and not having any formal Japanese education until college by which point she was already fairly conversational in Japanese from having watched variety shows and anime. We met while I was studying Japanese at college so I have a pretty good idea of where her Japanese ability stands.
Another anecdote, a Chinese friend of mine from college just passed the N1 with a perfect score. His Japanese education consists of a few classes in college, anime, and video games. He says although he thinks his English is more fluent due to him living in the States, Japanese was easier for him to learn.
The fact that it is a well-documented language that has evolved over thousands of years with almost no external influence and is entrenched with thousands of years of cultural concepts that are distinctly unfamiliar to a majority of the western world. Many phrases used in Mandarin today date back millennia. Also something that many people don’t recognize is that a single character can embody many meanings depending on the context. It’s not as simple as memorizing the character because you have to know which meaning a character is representing within a particular context.
I’m extremely skeptical of your claims that you learned Chinese grammar in a week as a Chinese learner myself and I’m willing to bet you don’t realize how much you don’t know. The Chinese grammar wiki has 505 articles on grammar split across A1-C1 levels of the European Common Framework for language proficiency. This wiki is also non-exhaustive. This isn’t even including the fact that Classical Chinese, which is a basis for many 成语 used today, has a completely different grammar than modern Chinese.
It's worth noting that the aforementioned list is the unified pronunciation list that was published in 1985 by the Ministry of Education. The reason why you see some words only having a single (unified) reading in that list is due to the necessity of having to unify them in the first place, although there are still quite a few words with multiple readings. Keep in mind that there was no official language of China until 1932. Without going into detail about how pronunciations evolved with the change of dynasties and how China actually has 300+ spoken languages, the need for a unified pronunciation stems from the fact that many people in China, historically and even today, do not speak standard Mandarin as their first language. In other words, prior to 1985 it was much more chaotic. If you want a more up-to-date comprehensive list of words with multiple readings (多音字) you can find it below (although this is not an official government list). I've linked directly to the common words of which there are 106 (although the page does not define what is considered "common").
While not as egregious as Japanese where characters can have 15+ readings, the number of exceptions certainly are not few. Below is a link to the official table of words with multiple pronunciations in standard Mandarin.
Another anecdote, a Chinese friend of mine from college just passed the N1 with a perfect score. His Japanese education consists of a few classes in college, anime, and video games. He says although he thinks his English is more fluent due to him living in the States, Japanese was easier for him to learn.
Point being? I think it’s subjective.