I'd advise that prior exposure to AOPS at the Alg or even pre-Alg level, and maybe also having actual live AOPS instructors (instead of a mom who winged it with lesson prep the day before, LOL), would have made it even more successful. Just a thought.
Point well taken!

I have a tendency to meld a hodgepodge of content for DS for math - he needs some basic skills and automaticity, some problem solving, some proofs, and some interdisciplinary application.
Re: lateral stretches - yes! I have my eye on AOPS counting + number theory program this year if DS runs through the existing material faster than I'd expected, as well as a grade 11 physics text.
For competitions + Olympiads - this has been a topic of discussion I've had in the DMs. DS is a slow, deep thinker who isn't overly motivated by the competitive angle of math or traditional outlets for mathy kids, like chess. Combined with his EF deficits, the format of competitions would actually (I suspect) be quite stressful for him. We tend to jump straight to the MOEMS / AMC problems in AOPS, and he can usually solve most of them in a few minutes. But there doesn't seem to be any intrinsic drive to do more of the same. *shrugs*
What I've been doing with him, instead, is watering down some first year uni linear algebra (simple stuff like Euclidean vectors, inverses, determinants, systems of linear equations), stats, introducing some philosophy / logic concepts, with an aim to teaching him some econometrics and decision theory modelling. There's a lot of fun application to be had with those topics.
He seems to like cryptography, so maybe some combinatorics next?
Truthfully, I have no idea where he's going to land career-wise. Instead, I try to feed him fun little problems or concepts from several disciplines, and see how far he can take them logically on his own steam. Then, to the extent I can, I throw the ball further afield and connect the interdisciplinary dots.
The common thread in everything is a hunger to find a generalizable solution and to automate the heck out of his process.