to the OP, if you want to look for inspiration, WWE professional wrestler John Cena was on Trevor Noah’s Daily Show a couple of weeks back. John can speak Mandarin fairly well, relatively speaking. A heck of a lot better than Mark Zuckerberg that’s for sure. Anyhow, he talks about why he decided to learn Mandarin, building a foundation for future generations, and getting out of his comfort zone. He’s also written a children’s book, which has one really good phrase of advice - “if you only do things you’re good at, you’ll never learn anything”. It’s very applicable here.
Sure, SAT or AP would be best, but I fear it would be difficult to get reasonable scores just from self studying with Duolingo, especially for someone without Chinese heritage background. The HSK has more approachable beginner levels and is still known at the college level. I assume the goal of getting a basic level certification would be less for getting college credit than to be able to document achievements from self studying.
Which goes back to my earlier point that language learning should be tied to a goal. In Cena’s case, he started to learn it to learn it after visiting China and wanting to know more about Chinese culture and as part of his desire to bring WWE to China. And, it took him 5+ years to get to his current level.
I think John was speaking pretty decently after a couple of years of practice, IIRC. But to your point, it’s a hard language to learn and to attain professional fluency does take years. But that’s probably the case for most languages.
However, some languages are considered more difficult for English speakers to learn than others. Chinese languages are considered to be in the most difficult category in the following listings:
https://www.dliflc.edu/home/about/languages-at-dliflc/
http://aboutworldlanguages.com/language-difficulty
My point was really that he had an underlying reason for learning the language. I didn’t. Which is why he succeeded and I gave up. I have no idea if he can read and write Chinese, but for him, it doesn’t seem to have been his goal. And that’s OK too.
Aside from one’s own ability, I think the mother tongue has something to do with it, as does the inherent difficulty of the target language. The Romance languages, in general, are fairly easy to learn for an English speaker. The East Asian languages, particularly when you include reading and writing, are amongst the more difficult.