We went a few yrs ago to Cala Longa, which is a very quiet resort in a nice bay. It would have been early-mid Sept but can’t remember exactly when.
There were a few people at the hotel who would sleep most of the day by the pool, get ready early evening and then head off for a night out and crawl back around breakfast time the next day.
We did a Sunset Cruise trip one evening from Ibiza Town and it all seemed to be in full swing although there were lots of posters up for end-of-season parties and final nights.
In terms of weather, I think we had one rainy day in two weeks but for the rest of the time it was factor 20 and sit by the pool in shorts.
It cooled down in the eveings, but was still short sleeves and shorts weather.