Hi there. I'm currently a secondary teacher and assistant head and I'm doing a part-time EdD alongside my full time job. I love doing both to be honest but there are significant differences.
The PhD would involve significantly more time working completely independently. Personally I really enjoy the face to face contact I have with students, colleagues, parents etc and love that part of the job. I also love the solitary work of reading research, reflecting on this smell looking at ways to improve my practice. I find doing both means I get better results from both as a result.
I suppose it would make a sifference what you intended to do the PhD in and whether you wanted to teach as part of your student ship to undergrads.
I think the transition is certainly doable and you'll be very good at managing your time but you need to think about what your end goal is. Why do the PhD? If it's hust a way out f teaching then you'll probably find it doesnt give you want you want.