Even Sanderson's "adult" fiction can read as incredibly immature at times. Every character seems to have a caricature of a mental illness. Every thought and feeling they experience seems to tie back to it, and it derails the story while taking away from the excellent world-building. I loved the Stormlight Archives to start with, but they became more and more "woke" (as much as I despise using that word) as time went on.

If you're interested in returning to fiction but through the lens of an adult - a little grittier - I'd recommend Joe Abercrombie, Pierce Brown, Dmitry Glukhovsky, Adrian Tchaikovsky, Dan Abnett, and Andrzej Sapkowski.