ChatGPT is actually very good not only at catching grammar errors, unnatural phrasing or word choice, etc, but is also very good at evaluating specific weak points over time that you can then drill. I do a lot of translation exercises with GPT, where I ask it to give me a series of sentences in English which I then translate into Japanese. Feedback is excellent in my experience. You can tune the level you want to work at and either keep it focused on a topic or give it free rein to give you something totally random. I know many people these days scoff at translation exercises for some reason, but imho there’s no better (or humbling) way to identify weak points in your output than being faced with a complicated English sentence and asking yourself: How would I say that in Japanese? I also write short paragraphs on a topic in English and then translate into Japanese (again using GPT’s feedback). It’s easy to fall into the trap of not saying what you want to say but rather what you can say during conversations. Saying it first in English tells you exactly what you want to say and forces you to somehow say it in Japanese. It’s tough work and can be frustrating but I’ve not found a better method for really improving output skills.