I very much enjoyed this report, and especially the fact of a significant target of your attention being the ZX Spectrum - this is a true and honest approach to the programming art!

:)

Its definitely true that even learning older architectures can sharpen your focus and skill, general platform dexterity. You may not use a lot of the Z80 assembly skills elsewhere (but then again; you might), but for sure the bounds of your depth and understanding of how computers work today are definitely reset. Sure, stay 'focused on Ruby'/et al. - but if you have to pace yourself at your day-job, having a fun project/waste of time in the same department albeit with significantly fewer bits is also, as in such an attentive case, a professional perk. Old computers never die; their users do!