if you definitely want to do PD probably including it a combination including maths and a science (e.g. physics) gives you most options for further study.
computer science = technology
Yeah, they technically count as STEM, but they're not real STEM. They're not what people typically associate with STEM. It's like saying humans are animals. Yeah, they technically are... But they're different.
Technically I think it does, it was always considered STEM at my school at least... Technology/engineering and stuff. But with the design element it is kind of one foot in, one foot out of the STEM door.
I think it depends on the specific subject and content of the course (I would personally consider Product Design: Resistant Materials to count as STEM, but perhaps not Product Design: Graphics...) Its about the Technology-Design balance.