I once wrote some deliberately bad pieces of fanfiction and fed them into various AI models to see whether any of them would call out that the fiction was bad. With the right prompt, you can get the AI to say that something is bad (it's more likely to admit something is bad if you tell it that someone else wrote it, and more likely to complement the work if you say it's your writing), but it takes a surprising amount of work to lead it there (unless you straightforwardly tell it the work is bad).
You can ask LLMs for suggestions on your work, but I mostly find the suggestions of all of the models to be bad. (I'm a huge fan of LLMs, so my pessimism here is coming from a place of thinking AI is "amazing in general, but terrible for this".)
Hyperwrite is the best model for helping me "tighten" my prose, as it often has good suggestions for how I can reword sentences to express the same thing with fewer words, though even then I ignore over half of its suggestions. Still, I highly value that kind of "never use a longer sentence when a shorter one will do" editing, and Hyperwrite is the only service I've found that consistently offers good feedback for those kinds of edits. (It's shockingly terrible at all other forms of feedback, but that type of editing is useful enough for me to shell out for their paid service anyway.)
If you want test readers, I recommend you join the Royal Road "Writer's Guild" discord server. They have a "Critique Circle" channel where you can join other writers and all agree to do test reading of each other's works (usually 10k - 30k words). You and 3-6 other people will each read everyone else's stories and offer in-depth feedback. It's time consuming (because you have to read X other stories in order to get X other people to read yours) but I've had great experiences with it. Not all fellow authors will offer useful feedback, but if you join a circle with 3 other authors then odds are at least 1-2 of them will have great suggestions.
