Your English is excellent - I wouldn't know it was a second (third?) language for you.
FYI, "mostly perfect" is problematic, as "perfect" is an absolute - something is perfect or it isn't perfect. Though it could interpreted as "most of my English is perfect", which makes sense. And there's also room for artistic license.
Many native speakers say this kind of thing all the time with "unique", by saying things like "one of the most unique" thing. So you're in good company, you speak English as well as many native speakers! 😁
(It's a minor thing, I doubt most people notice that qualifying an absolute doesn't make sense).
Again, your English is excellent - I really wouldn't know otherwise.