Here's a test for your team: can a new developer deploy to staging on their first day?

Not "can they ship a feature." Can they clone the repo, run the tests, and deploy a trivial change? If the answer is no (three days of setup, credentials from someone who's on vacation, a wiki page that's two years out of date) that's not an onboarding problem. That's a development environment problem that everyone on the team has learned to work around.

I've joined teams where I was productive on day one, and teams where I spent a week fighting setup. The difference was never the complexity of the domain. It was whether someone had bothered to make the path from zero to deployed a straight line.