When you're just starting out, mainstream is excellent. Nothing wrong with that, rather that is what you're supposed to do. Go with what everyone else loves too and you'll usually enjoy it. There's a reason it got popular! Here are some of my personal favorites that aren't already on your list, that are also all mainstream. For your convenience I separated them by the streaming platform they are available on.
My top 3 Crunchyroll favorites:
{Re:Zero} - Isekai where the MC dies in video game-esque time loops... a lot. Made by same studio as Steins;Gate, White Fox.
{Konosuba} - 4 idiots in an fantasy isekai world, parody/comedy
{Attack on Titan} - Can't get more mainstream than this.
My Amazon Prime Video favorites:
{Made in Abyss} - Increasingly dark show about 2 kids' journey to the bottom of a giant abyss.
{Houseki no Kuni} - One of the best "CGI" anime I've ever seen, probably one of the best ever.
My Netflix top 3. These ones probably fit your preferences of "superpowers" and "high school" the most:
{One Punch Man} - Action/Comedy/Parody, about a superhero that beats everything in one punch, has great animation
{Little Witch Academia} - "Magic High School", the closest possible thing to Harry Potter the anime.
{Puella Magi Madoka Magica} - "Magical girl" show with a twist, i.e. it gets progressively darker over time. (middle school but close enough)
If you want to give Slice of Life another shot I recommend anything by Kyoto Animation. My own personal favorites from them:
{Hyouka} - Slice of Life with mysteries, slow burn with a great main 4 cast. Available on Crunchyroll.
{Violet Evergarden} - What does "I love you" mean? Available on Netflix.
{Hibike! Euphonium} - High school band with yuri sub-text. Available on Crunchyroll.
{Nichijou} - High school comedy. No legal streams unfortunately :(
{A Place Further Than The Universe} - 4 high school girls go to Antarctica, dark horse smash hit of last season. Available on Crunchyroll.
{Shirobako} - Meta show about an anime studio making anime. Available on Crunchyroll.
Also here are two good movies that are set in high school
{Kimi no Na wa} - has supernatural element like Kokoro Connect, body switching
{Koe no Katachi} - Made by KyoAni