I have a chronic illness that sounds very similar to yours, which caused me to miss a lot of school in high school too. I was also looking for a small school with minimal partying atmosphere.
I go to Amherst now, which I agree would be a reach for you, along with Williams and Wellesley (which was also one of my top choices). If you get your SAT up a bit, you’d be in a more comfortable position.
If you’re interested in Wellesley, also look into Bryn Mawr and Mount Holyoke, both beautiful schools with great academics but a little easier to get into. Other schools you might like: Allegheny, Susquehanna (has business administration major), Earlham (if you are willing to go a little further from the Northeast), Muhlenberg (previously mentioned).