I did do a search … besides trip insurance (hurricane season!) & possible closures, are there other downsides to visiting in September? Any upsides?

We went to SXM several times in September, and recently in October. The upside is that there is usually very little traffic, either on the roads or on the beaches or restaurants. To me, that is a very BIG upside. Also, planes are usually a lot less crowded.

Some residents and locals leave the island that time of year.
Therefore some of the restaurants and businesses are closed.

Drphilsxm

We often celebrated our September anniversary on SXM and the only thing we ever noticed besides the decrease in traffic was the fact it often rained at night but that never had bearing on our daytime activities. Some restaurants will be closed but trust me, you won’t go hungry and you’ll still have lots of options to choose from.

Most of the restaurant closures are on the French side during that time, but you will definitely NOT starve to death!

What I remember was the humidity is way up. Coming from the West Coast that is a big change compared to what we are used to.

We are here now and traffic is pretty much nonexistent. Going from point A to point B is easy peasy.

We've visited many times in September. There are some restaurant closures but not many. The weather is lovely, the water very warm and fewer visitors.
Last October we spent 3 weeks. Whole different ballgame. Many, many closures which included lots of our favorites. The weather was hot and stormy. No, we didn't starve and still had a great time but we'll avoid October and probably most of November in the future. It's the slowest of the low season. But I wouldn't hesitate to go in September.