I got a 36 on the ACT (36/36/36/36) and an 8 on writing. I’m really not happy with my writing score, as it definitely doesn’t reflect my ability as a writer. I’m looking at super selective colleges like Yale/Harvard etc., so is it worth retaking the ACT to improve my writing score? Will an 8 hurt my chances, even minimally, at admission to selective colleges? Also, would it still be better to only send in my SAT score, which is comparable in composite but much better (22) on the essay?
No. Move on to other aspects of your application.
If money is not an issue, send both.
Relatively few colleges (28 according to Princeton Review) require the SAT/ACT with essay. Even for those schools that do, I would posit that your writing in your essays would be more closely evaluated than your ACT/SAT essay scores. Harvard basically gave this as a reason, although not the only reason for eliminating the need to take the ACT/SAT with essay.
http://www.thecrimson.com/article/2018/3/20/harvard-drops-writing-scores/