> J-1 after it expires (1.5 years)

1.5 years might be for your case. For research scholars you can extend it for up to 5 years.

> But transitioning from a J-1 to a H1-B doesn't require waiting that much. I was out of the country only for 1 month or so.

The current processing time for an H1-B visa is eight months. You also aren't required to leave the country.

I'd be careful taking your experience and extrapolating it to all situations. People here may be mislead by some of the information you provided.