Isn't the design of the game that you HAVE to do the side quests to level up for the main quests?
Nope. Leveling isn't a problem at all. I fully watched Skill Up review yesterday after I was done with the main story and while I agree with the video title and many many points he said, I was honestly surprised he was under leveled so much for the last bonus area.
The problem is the main story itself drags with fillers that are unfortunately main quests, and if you dodge them at one point you will have to complete them later to progress the story and open the new areas.
Some story arcs should have been optional side quests lines and that would have been much better.
It's weird. The story itself is much shorter than both Origins and Odyssey but it is stretched by fillers.