I have a small window of opportunity to go windsurfing in the Med or North Africa in the the first 2 weeks of October 2026.
At first glance The Canaries seem the most reliable.
Has anybody had any experience sailing that time of year?
Vasiliki perhaps?

Vassiliki is thermal summer breeze, no (reliable) wind in october.
Canaries are a bit of a gamble now, lowest wind probability of the year between september and november.

Egypt (Dahab) can be good.

Cape Verde is further south than Canaries, and does not have those autumn doldrums.

Be aware. My above remarks are all wisdom from the pre-climate-change era.
Passat (Canaries); Meltem (Turkey); Meltemi (Greece) and Northerly (Red sea) are less reliable.
Especially around the start and end of the periods when they used to blow.

Gruissan and La Franqui are still a VERY safe bet. Not warm anymore, but windy.
If you need to fly, try Perpignan.