One might think that chess would qualify. People become grandmasters in their 20s, with less than 20 years total experience, so surely if you quit your job and focus on only chess from 30 to 50, you can gain enough skill and experience to compete at the world level...

And yet, in practice, almost all chess grandmasters have started young. I don't know of any grandmaster who started playing chess seriously past 30 (and chess.com didn't have any concrete examples that I saw: https://www.chess.com/forum/view/general/gmsims-who-started-...)

Physical sports are obviously out, as the world level is dominated by people under 30 for all of them.

I'll also comment further on two things you wrote in your original question:

1. "compete at world level" and 2. "don't have to be your full time thing"

In practice, the majority of things that have world competitions will also have people who will devote themselves, body and soul, to it, so if you are intent at not spending all your time on this, not making it your passion, then you should not reasonably expect to "compete at world level".

I think you might do better by classifying "compete at world level" as being very broad, like considering writing a "new york times best seller" to be competing at writing at the world level (in which case, yes, some authors started after 30). If you broaden it to simply "be able to have fun", I think you'll have a far better time of it, and at that point almost any physical activity now becomes something you can start past 30.