You can write by hand incredibly complex kanji that the average Japanese person can’t like 薔薇, 醤油, and 欝. You know the readings to very difficult Japanese words that are rarely used. You can pull out random old ことわざ (Japanese proverbs) perfect for the situation. You know the meanings of 4000 kanji. So what do you have a bad tendency to do? Take every opportunity you can to display these excelled abilities to Japanese people.
Having these abilities is great, especially if these were some of your goals. But you must remember these important words: Don’t show off.
One of the biggest tell tale signs that your Japanese isn’t that good is that you perform these strange feats of knowledge to demonstrate your Japanese prowess. Japanese people don’t care about this, and for the most part, don’t like hearing this. It makes you sound cocky. It is a sign to them that your Japanese really isn’t that great. Japanese culture is about being humble and you want Japanese people to like you. I know you’ve watched anime before and I’m sure you have noticed that all the best characters in all the best stories have power but don’t reveal it until they have to. Those who like to show off power are the ones who don’t really possess it and eventually get beaten.
It can feel good to occasionally hear a 日本語が上手ですね (you’re Japanese is great). But this praise is often shallow. After hearing this enough you probably know what they really mean when they say this. If praise is a positive motivator to you, which there is nothing wrong with, there is a best and natural way to show off your abilities: just be awesome. If your Japanese is good, it will show itself off. No direct effort is involved. Talk in a smooth accent and intonation on a wide range of topics, add humor to your conversation, and make sure whoever you are talking to is enjoying talking to you.
Being impressive while not trying to impress is the most impressive. Become a real master.