I don't think there is just one thing that I wish I had learned. It's more a matter of recognising that the PPL experience was a little sample of most things, without in depth experience of many of them. I didn't do much in full ATC airfields, for example. So i just recognise that I need to build that experience.
I never had an SVFR experience so thats something which is on my mind for getting some experience. But I have no idea whether most PPLs would get experience of an SVFR clearance.
Flying over mountainous areas is another.
And, of course, the whole notion of going foreign isnt something one does at PPL. Well, I didn't.