I do think that new VOs where you can imply this concept, and altering the opening crawl, are solid ideas. Although I think the biggest problem visually (outside of altering Starkiller Base) would be the First Order’s new gear and polished look. New ships, new armor, new weapons, not exactly what you would expect from a ragtag terrorist group.

You could just describe them as a small Imperial remnant maybe. So they have some resources, but they don’t have a massive fleet or anything (which could make this hidden fleet in Episode IX more important to the First Order).

I think another radical idea I’d like to brainstorm more would be making Starkiller Base a Star Forge-type device that allowed them to rebuild their fleet. It’s another interesting alternative to help explain how they were able to rebuild their military in secret.