I certainly have done my time with writing CRUD apps, but for the last decade or so, I've primarily focused on distributed systems stuff (usually involving Kafka or ZeroMQ or something), which is a bit more architecturey and I occasionally get to do some fun concurrency theory.

But when doing the more boring CRUD stuff I historically have gotten my math kicks out of playing with functional languages. I got into Haskell pretty early because I liked the idea that the type system felt pretty mathy.