Only tangentially related, but if you're looking for a sufficiently challenging development project the iOS MDM protocol is a great one. It's well-documented,

Only tangentially related, but if you're looking for a sufficiently challenging development project the iOS MDM protocol is a great one. It's well-documented, so you're not stuck banging your head against a wall. It lends itself to microservices very well, and is explicitly asynchronous for a lot of operations. I spent a week or so writing one in Java (and later ported to server-side Swift) and found it moderately challenging and a lot of fun to write.