Hey @Steph.priest3105, Thanks I had the best time, almost a year since I left :(.
My trip finished early August. Weather wise my trip was pretty hot (40 Degrees C) all the way around from Paris (bar switzerland) until we were in Krakow and then it was jumper weather all they way back to London. Going a bit later in the year like you are I assume you wont have as many hot days as I did and more cooler days. I took 2 jumpers with me and 1 thermal top but didnt really use them much.
I would recommend you pack for both types of weather (which sucks). It will be pretty warm around Nice, Italy, Greece, Turkey, Croatia at that time of year and cooler more north. Instead of bringing big jackets which take up a lot of room maybe bring one jacket and then lots of layers you can wear underneath to keep you warm. I had a quick google and the colder places will only be around 20C at that time of year. So basically just pack for mid range temperatures but have an option for when it may be cold.
Hope that helps!