You can use
https://hemingwayapp.com/ to check for passive voice, adverbs and complicated sentences.
If you are willing to work in return for the own review of your work, you can always try
www.scribophile.com.
Scribophile is intended for authors to critique each other's work, and they are pretty gung ho against using AI there, so you'll get at least three people reviewing your chapter for grammar, sentence and any other advice they come up with.
But it does need a lot of effort on your part as well. To post one chapter for critique, you need usually to critique 3 to 5 other chapters in return, depending on the amount you write. And not all the feedback you receive is good, given some people just want to get the points to get their own chapters reviewed and don't really care about improving other's work.
And if you don't want to sub in the website, you can only keep two chapters at a time. Which shouldn't be a problem, really.
As one last thing to point out, the people there are usually more interested in formal publication instead of webnovels, but it is still a great way to improve your writing, with people pointing out to you things to improve.