Ruru21

Personally I started with Da Capi 1 and to be honest, despite all the translation irregularities and dated graphics due to it being such an old game, I actually found it quite enjoyable. The story was actually interesting and so was the soundtrack. Yeah even the characters were pretty good and it gives us a firm foundation in the Da Capi universe.

Yes as you said, Yoshiyuki from Da Capo II learned it from Junichi from Da Capo I. The protagonist from Da Capo III was the original creator of the Wagashi Creating spell since the main story takes place far in the past before Da Capo I even. Kiyotaka and Ricca Yoshino were the grandparents of Sakura and Junichi of the original Da Capo VN, were the ones who originally planted that Everlasting Sakura Tree that grants wishes, and Ricca aka Granny Yoshino taught Kiyotaka's spell to create Wagashi to Junichi. So, in all honesty Da Capo III sets the stage for Da Capo I in a very big way.