The simplest useful tool here and the one that costs nothing. Paste your text and it highlights the sentences that are hard to read, the passive voice, the adverbs propping up weak verbs and the words with simpler alternatives, then gives the whole thing a reading grade. It teaches as it goes, so most people need it less after a few months of using it.
It is a readability checker rather than an AI writer; the generative rewriting is a paid add-on and not the reason to come. But for cutting a draft down to something people will actually finish reading, the free web version does more for the quality of your writing than most paid tools.