'reading a defense' boils down to
1)reading defenses pre-snap (usually what is referred to as 'reading defenses')
2)making adjustment based on #1 and your experience/film study
3)ALSO understanding where defense is on the field during the play POST snap and how are they going to cover your throw and all, ESPECIALLY if your read of #1 was wrong
all these 3 things combined are EXTREMELY difficult for any qb in the league. as someone who have watched a ton of film as a hobby, Id say theres maybe 3 active qbs who can read defenses to be good in these three things. and even that might be a stretch.
I mean, theres peyton manning who is by far the best reader of defense ever, and theres everyone else. theres tons of qbs in hall of fame who werent that amazing at all aspects of reading a defense. a lot of them were awesome at the reactive part of #3, had great vision and decision-making, but didnt necessarily have the best reads or made the adjustment pre-snap OR post-snap
black qbs in general (fukking hate to generalize here) arent good defense readers, but that stems from how they are being used in college and high school. no one bothers to hone their passing and defense reading skills, and coaches just exploit their athleticism.
theres no way you're gonna be a great defense reader if coaches make you run the read option a ton and simplify offense for you in order to win short-term. you can be a good qb but you wont be able to read defenses that well.
finally, like Ive said, reading defenses really well isnt necessarily a prereq for being a great qb. being real good with your decision making is usually more than enough. overthinking things can be an actual side effect of trying too hard to rely on your reads and chess-like strategy of thinking several moves ahead with different possibilities.