I'd like to make a confession now

I believe the easiest switch would be from Spectrum to CPC. With some experience at both platforms I could even maybe one day try to decompile and port some game from CPC to Spectrum

Unfortunately I never did it. I can see two obstacles - graphic modes of CPC which are quite badly documented (or at least Google failed me here) and different tools - emulators, debuggers, decompilers, art utilities, graphic rippers and so on.
So I'm really happy to see your tutorial. Don't hesitate to write about CPC. We know Zx Spectrum and I suppose there are people like me who would like to learn something new.