I agree with you that all on the same day does a disservice. However this piecemeal primary process over such a long period is also a disservice to the voter. My personal take is for a regional primary system, say 6, over a 3 month time period followed within a couple of weeks by the convention. Ideally the whole process, primaries, conventions and general election could be compressed into no more than 6 months. It is emasculation for the candidates as well as voters to have this thing stretch out for well over a year and does absolutely zilch in the way of finding the person who will win.
Another suggestion is to have meaningful debates held by other than self serving “news” organizations. Debates that ask substantive questions and a moderator that would not allow any one individual to avoid a direct answer to the question, answers with some meat on them.