No it's not. I studied Japanese as my fourth language for 1.5 years and went through pretty much exactly what the OP described.
Edit: Other languages were Spanish English and Italian.
I don't know what exactly the OP means by "vanilla" in this context, these languages are anything but vanilla! One could argue that perhaps English grammar is vanilla because it has developed certain creole-like characteristics due to various pressures during its history: Compared to most other languages in the IE family, its grammar is quite simplified, it's lost most of its complexity.
Japanese: topic instead of subject marking, deference forms, completely different morphological categories?
Chinese: productive verb serialization, very different syntax?
That's just off the top of my head
But that's not the point really. Learning any language well is mind-bending for any speaker of another language. It does help if the languages are different (I remember very vividly the click that went in my head in high school when I was taking English, Chinese and Hebrew simultaneously... that's how I ended up in linguistics grad school), but different people have their minds bent by completely different things. I mean, root-and-pattern morphology is really cool when it's that regular, but if you think of it it's not horribly different from Germanic strong verbs (write-wrote-written). Weird agreement patterns? Well, Welsh only uses the 3pl verb forms with a pronominal subject, so "they they-went" but "people he-went" (as, for that matter, do some varieties of English). And so on.
I mean, sure, whatever floats your boat, and it's awesome that studying Arabic can be so fulfilling; I've had the same experience with Welsh. But at the end of the day all languages bend your mind, and I don't think there can be an objective metric. They are just hard.
Putting Chinese and Japanese (I guess because their
writing systems are kind of similar) is similar to,
similar to saying the same for English and Basque (which,
like Japanese is agglutinative) because they both use the
Latin alphabet.
Actually, Chinese and Japanese people could read (or rather, infer meaning from) texts the respective other has written if they use the subset of characters that both know (Hanzi ∩ Kanji, if you'd like ;) ), even though they could never communicate verbally. It'd be a bit like showing each other drawings of things that both know and understand, yet have completely different names for.
Comparing both on a purely grammatical/syntactical level, of course, is pretty meaningless.
I've been studying Japanese as my fourth language for over 2 years, and I have to disagree with both you and the OP. If you didn't find Japanese to be very gramatically challenging or interesting, you unfortunately probably didn't get into any keigo, which contains grammar of a mindbogglingly high level of complexity that is probably only surpassed by a similar system in Korean, since Japanese grammar was artificially simplified during the Meiji Restoration in order to make a simpler national dialect that all Japanese could easily learn/use. Nevertheless, the situational shifts in Japanese grammar based upon the relationship between the speaker and the listener are very difficult to become accustomed to, even for native speakers.