The various R packages produced by the MOSAIC team.
Even within the 500 words, you can generate a “Summary” report that shows your overall readability score, passive voice percentage, and overused word density. This gives you a high-level diagnosis of your writing health without fixing every comma.
However, if you are writing for pleasure, or you are a student on instant noodles, the free version is a legitimate, permanent option. prowritingaid -free version-