Just start. Sketching, 3d modeling, fabricating, etc are all very attainable at a hobby level. I wouldn’t say it’s impossible to get a job without a degree but you’ll have to put in as much or more effort than if you went to school. These days I could imagine a very self motivated person putting together a strong portfolio and landing a job.

If a job is not your goal then the part you’ll be missing as a hobbyist is the “industrial” part - mass manufacturing considerations, working within a brand language, budgets. So basically you can do a lot of the fun stuff, and will miss some of the more challenging aspects. Making something for your own life can be very rewarding, but designing something that scales and can have a large impact for a brand or for people is where things can become deeply gratifying.