Watched my little brother's handwriting go from total chicken scratch to actually readable in 6 weeks
His 4th grade teacher switched the class from those fat pencils to plain golf pencils in March and by mid April I could actually read his birthday card, so what small change fixed something like that for you?
My buddy had the same thing with his kid but it was the opposite direction, the teacher made them switch from pencils to pens. Something about how you can't erase so you slow down and think about each letter before it hits the page. Kid's writing got way neater in like a month. Wild how something that small just flips a switch in your brain.