> About a year ago, all three big AI vendors could end up with strange "corrections" that would throw you off, but not anymore.

Without a history of diffs created by this method, I don't believe you.

> I am decent with English grammar so most of what needs fixing is typos I didn't spot or misplaced commas and periods inside/outside of quotes (I always screw that up without thinking—even though I know the rules! LOL).

The rules there vary by style guide and are not objective.

LLMs seem like massive overkill for something the red and green squiggles can already accomplish.

Good writers break grammar rules when it helps convey a point, emphasize an idea, or just makes the writing more interesting. You are completely missing out on that.