According to PrepScholar with the Ivy league colleges , half require the ACT Writing section (Harvard, Princeton, Dartmouth, Yale) and half leaving it optional (Columbia, Cornell, University of Pennsylvania, Brown).
I personally wouldn’t stress too much about a 8/12 on the writing section and I certainly wouldn’t retake the ACT just to get a 9 or 10 on the writing section. I would spend the time on other sections of your college application like essays, continue to get strong grades the first half of senior year, make sure to apply to many match and a few safety colleges, and lastly TRY to enjoy your last year of high school. I’m sure you will have some great college choices come spring 2020.