I think engaging with works of fiction is just as important. Like anything, if a work of fiction engages and challenges you, and you are an intentional reader, it exercises very important muscles.
fiction absolutely can be intellectual. 1984, 451 Fahrenheit, anything dostoevsky or heinlein.
in fact, by adding that intellectualism is what makes these stand out.
But i do specify non-fiction because I wouldnt say most fiction is intellectual; or if you try to approach some fiction you'll quickly dig deeper than what's actually there and then it's just you superimposing.
The example i like is colour metaphors. Shakespeare will say that a character put a green shirt on. You're supposed to say 'thats just a new shirt, not the colour green' but no. It actually really is just the colour green. You cant dig too deep on most fiction.
Non-fiction slop also exists. You can just not read the slop. Yeah, you probably aren't getting much out of reading generic fantasy novel with self insert protagonist #12373838 but you also aren't getting much out of generic self help book with surface level advice #1537484. I'd argue fiction sticks better and brings more empathy and feeling behind what it tries to teach than non fiction does usually but is best for teaching things through situational understanding. Not everything condenses well into that format though.
Read lots of non-fiction. Whatever interests you.
Try to find overlap over the different interests. Try to find new thoughts there. You might be the first to find them.
Assume everything you know is wrong. It's generally true of >50%.
Hard is best, too hard is bad, too easy is bad.