Thank you everyone for your invaluable advice and your best wishes!

Ross Greene is amazing! Thank you for mentioning it! I'm reading the book again. It is indeed very easy to forget that DS is going through some unknown/unseen struggles because he appears so capable. I've had some success trying the suggested techniques to figure out why he misbehaves/what he's struggling with. I need to bring his teachers up to speed on Ross Greene. His teachers insist on giving him "consequences" for his misbehavior, despite me telling them it wouldn't work. My son now downplays (do I use "lie" on a 5 year old?) his misbehavior, e.g. instead of stating that he (intentionally) threw something, he would say, he couldn't get a good hold of it and it was an accident.

The automaticity comment is very interesting. I do notice my husband also has weaker automaticity than I do, despite having generally better memory than me. He can remember details from a book he read many years ago, but can't remember the route we took a few weeks ago to the same place, or how to operate an appliance that we haven't used for a while. He probably also has lower processing speed like DS. Whenever I suggest something new, his first reaction is usually "no" (just like DS too, lol) and after a while my words sink in and he'd probably change his opinion. DH also reads slower than I do, but has a much deeper understanding of the book once he's done. I could perhaps spend the same amount of time reading the same book 3 times, but wouldn't gain any additional insight, lol

I took your advice and had his vision checked at a university pediatric eye center. Luckily his vision seems fine and his eyes are tracking together.

DS has gone through an OT evaluation. Three tests were done but no direct OT service was offered.

(1) Bruininks-Oseretskey (BOT-2) was administered. He was 99th percentile for Fine Manual Control, but only 38th for Manual Coordination, specifically his Upper-Limb Coordination (measures visual tracking with coordinated arm and hand movement) was about a year behind.
(2) He passed the Shore Handwriting Screening. Letter formation was adequate.
(3) Sensory Processing Measure: he appears to have hearing and touch sensory issues, but doesn't exhibit sensory seeking and avoidance behaviors.

I came across a proposed diagnosis as a subset of autism, and the description very much looks like DS!
https://www.autism.org.uk/about/what-is/pda.aspx

I'm prepared to ask for an independent evaluation at some point and I'm hoping to find a psychologist who's familiar with 2e (found a list on SENG), and go from there. It is amazing how many PhDs have already seen him/been working with him. I'm actually very grateful and know that in a way my little guy is very lucky.

Happy holidays everyone!