Yeah, I agree. An open-source LLM-based grammar checker with a user interface similar to Grammarly is probably what I'm looking for. It doesn't need to be perfect (none of the options are); it just needs to help me become a better writer by pointing out issues in my text. I can ignore the false positives, and as long as it helps improve my text, I don't mind if it doesn't catch every single issue.

Using an LLM would also help make it multilingual. Both Grammarly and Harper only support English and will likely never support more than a few dozen very popular languages. LLMs could help cover a much wider range of languages.