Congratulations for beating FFIV! "A ton of side ultra-bosses", yeah, that's what the Lunar Ruins mostly are, I guess. However, think about all the US FFX fans who are angry about not getting to fight the Dark Aeons (I found the Dark Aeons more a nuisance than a welcome addition since they block your way to loads of places, but meh). If you want to do some side quests in FFIV and fight some more optional bosses, I guess FFIVA is a good choice for you.