Nice insight, Ashley!
While I agree that Zero's story is overly serious, it is still compelling and interesting, especially with the ultimate fate of the characters and how the story ties into the X series. If any series is melodramatic, it is the X series, with a barely there plot, recycled from each game to the next.
Just think of how Squid Adler in X5 is like "I will avenge Launch Octopus!" Sure took him a long time no? Probably the best things in that series is the X VS Zero fight in X5, and that's mostly because of the music!
As for the Zero series being hard, I've never understood that. They are actually easier than the X series and especially the originals, mostly because of the Cyber Elves and how powerful the Z-Saber is. The boss patterns are very simple and with practice, it is easy to do no damage runs!