If you keep your college GPA high, you’re a competitive applicant pretty much anywhere. Transfer admissions can be random, so I’d apply to a few schools that really interest you. This doesn’t mean pick every T15. Find maybe 5-10 schools that truly pique your interest and seem to “fit” you. Schools in transfer admissions are mainly looking for interest, growth from high school, and fit. Essays and personal statements are VERY important for transfers. Schools would rather admit students with lower test scores and GPAs if they have stellar essays and articulate good reasons for transferring and how they fit into the school they’re applying to. Not saying somewhere like Northwestern will take unqualified applicants, but the difference between a 32 ACT 3.6 college GPA getting in over a 35 3.8 could be fit and essays.
Retaking the ACT could be beneficial; however, test scores as a junior probably won’t be very important if you have 2 years of great college grades at a competitive school. If you want to retake it once just to see what happens, it may give you a better chance at some of the ivies, but it’s nowhere near as important as it would be for freshmen.
The other issue is that you may have to spend an extra semester or two in school because of credits. Some schools have strict policies on credit transfer for major classes. You may only have to do 4 semesters, but just be aware that’s the best-case scenario. Would you be willing to spend an extra year in college (and more money) just to go somewhere like Vandy?
I would also run NPC’s at all schools you’re interested in. See what the estimations say. That might narrow down your list of schools. You can also look at a list of loan-free schools to help you. They’re most often going to offer the best FA.