Bee listening to a bit of Bryce Dessner and Jonny Greenwood compositions lately. Both of them being from bands I like but don't often listen to anymore I really enjoy their more classical compositions these days. Greenwoods work with Penderecki is really good as is Aheym.

Any other mainstream/indie/etc musicians I should check out more contemporary classical works from?

Dessner:
https://www.youtube.com/watch?v=1Re4lgVCwGg

Greenwood:
https://www.youtube.com/watch?v=NnNN1KigLO0