I'm sorry they got you so down, especially because they are wrong.
Your English writing is really very good. I have never once noticed anything strange about your syntax. When I read what you write in English, I get the feeling that English is a very comfortable language for you. There might be tiny things here or there that give away you are not a native English speaker, but I want to emphasize that these are very minor, and very infrequent.
I have a strong suspicion that the people on that other forum are incredibly self-conscious about their abilities in English and are attempting to cut down everyone else as a way to make themselves feel better. This is a really common human trait, unfortunately.
Language is first and foremost about communication, not perfection. We, native English speakers, do not write perfectly - not even close! So why should people using English as a foreign language be held to a higher standard than natives?
Every time I read something you've written, I am continually impressed with your abilities. I don't think I will ever have such abilities in the languages I'm learning. And I do agree that reaching a C2 level is very different from speaking like a native. Those exams test very different skills. Many native English speakers would not be able to pass a C2 exam in English. And I think that plenty of people who can pass a C2 exam would have a "deer in the headlights" reaction if instead of writing they had to instead converse with native speakers, especially ones with regional accents. From my understanding, the C2 exam lasts 4+ hours, but only 12 minutes of that time is spent on talking, and that 12 minutes is probably not using a lot of colloquial language and slang.
Since you are also learning Italian, I am going to link an article that describes in Italian something that I think is very relevant to your situation with these people on the other forum (Il Camion della Spazzatura/The Garbage Truck):
https://www.efficacemente.com/felicita/vivere-felici/