Generally the thunderstorm season of mid-September begins to ease off, early October can be summer-like without the searing heat. So the dates you will visit are pretty much like mine, and I prepare for everything, from hours of hot sun to a day or two lost through storms. As alluded to, the sea is still warm.
Best to keep an eye on the rain radar and sat24.com satellite view just in case as that shows the storms brewing up.
This is the weather Ibiza had end Sept 2022
Weather reports from September 2022 in Ibiza, Ibiza, Spain with highs and lows
www.timeanddate.com
2021
Weather reports from September 2021 in Ibiza, Ibiza, Spain with highs and lows
www.timeanddate.com
2020
Weather reports from September 2020 in Ibiza, Ibiza, Spain with highs and lows
www.timeanddate.com
I'll take that any day!!