Of the Apple Intelligence writing tools that were released in beta recently, I’ve found the ones which try to rewrite your text in any way to be pretty bad, at least for me. The one feature I’ve been using many times a day, however, is “Proofread.” It’s sort of between macOS grammar/spellcheck and Grammarly. It’ll see where you missed a comma, fix capitalization, and that sort of thing without changing the tone of your text. It’s even smart enough to not mess up your formatting when writing in Markdown.
In fact, I like it so much I set up a keyboard shortcut for it. If you don’t know, you can set up keyboard shortcuts on your Macintosh for any menu item. I made mine control-command-p. If you’ve never done this before, Apple has a support page that will show you how it’s done.