I'd like to think it comes down to my expertise, but I don't believe I did a lot of designing.

Sure, I've given a lot of inputs over the time, probably I've steered it to solutions that work.

Maybe it was also helpful that I set up a skill to use a different model to review the code for correctness, technical debt, and whether the location where the code is added is appropriate or warrants a new file or abstraction. I set this up long before subagents were widely used.