I want everything to be owned by myuser, group media

Wait, “everything?” Yeah, that’s probably contraindicated. You don’t want to be changing ownership of stuff in, say, /etc or /bin or whatever to your user. For the most part, stuff in those locations should be owned by root:root. If there are exceptions (things that should be owned by root:<something else>), the package manager will make sure they’re set as they should be.

Whew. My 586 is safe.