I went back and tried the quests ...
Ichigo's and Macho's quests were good. They made me explore a little and find a good place to grind for both. Opeo's quest is funny and a good katamari reference. I haven't cleared Ace's but a harder version of the first battle is a good idea. I may try some more but for now I'll take your word for it that it's possible. I don't know the moves very well.
All in all the side quests are cool, and having each party member/obtained Cousin ask for a quest is a good idea.