I wouldn't say it was very difficult, but it did take quite a bit of time. Apart from some basic principles (no GC, no RAII, "everything is an expression"), I basically kept adding features whenever I hit some pain point trying to write actual programs in Alumina. If I were to do it again, I'd probably be more methodical, but anyway, here we are :)

Protocols were probably the trickiest feature of the language to figure out. As for the compiler itself, surprisingly, the biggest hurdle to get over was the name resolution. It's a tiny part of the compiler today, but everything else was much more straightforward.

I don't have formal CS background, but I have been coding for a long time. I read the Dragon Book and would recommend it to anyone writing a compiler, even though it's a bit dated.

I don't know Racket or LISP myself so I cannot comment on that part.