@Pzaz99 - Lots of wonderful advice on this thread. DH and I became guardians of our niece for her last two years of high school. She moved to our home; though we were close all through her life, this was a huge transition. All agreed the guardianship was the best way forward, yet there was grief and anger at the circumstances, as well as the challenge of a different household with adults at the helm.
Your niece is in good shape academically. You are showing her what matters to you with every question asked and every issue you nudge to the foreground. Focusing on getting to know her by listening and following her lead, noticing her interests, and how the days are going (without achievement oriented expectations dominating) will help build what our niece aptly labeled “the firm surface to push off from” she needed as an emerging young adult. You don’t have to know everything, yet can help her access support and resources as indicated. By way of encouragement, our niece has made her way into her mid-thirties with a wonderful husband, several well-tended kids and pursuit of her goals. We remain extremely close. All the best.