What's the least painful way to become an interior designer?

Depends a lot where you live. In a handful of US states and some other countries "interior designer" is a regulated title that needs a degree, work experience and an exam, while "interior decorator" or "stylist" is not. Check that before you pay for anything. The least painful route for most people: a shorter accredited course or diploma rather than a full degree, learn SketchUp or a CAD program properly, and build a portfolio from your own place and friends' rooms with honest before/after photos. Then get a foot in the door at a furniture showroom, a kitchen company or as an assistant to an established designer. That's where you learn budgets, suppliers and clients, which no course really teaches.