It seems like it should be possible to create the necessary files to create an override package that would replace all the base game buildings and townies with custom ones. But my attempt to override one townie's wardrobe was a failure and I don't know where I went wrong. Shouldn't it just be a matter of pointing the main build file to the desired lot/townie ID numbers? The end goal being to have every new save start with custom buildings populated with custom sims so that I don't spend days rebuilding the world every time I start a new save. I'm aware this would be a fairly large task.

Well, I've made progress on this front. I now have completely custom sages in the magic realm that are built automatically in place of the default sages. It was as "simple" as I thought, just redirect the files like any other override. But it took a bit of work still. Mainly, I found out the hard way that the traits in the Household Template have to perfectly match the traits in the Sim Template. Otherwise your new sim will not work. If you're just doing a makeover, this doesn't matter because you aren't messing with traits. It becomes a hassle when you are changing to completely custom sims.