Ego didn't feel like Galactus because though he was basically making the world himself, they focused the storyline on his personal relationship with Starlord and 95% of it was on his planet, so the epicness was lost in favor of making it more personal. In GotG2, it worked. But, Galactus can EASILY be seen as a viable threat. You just have to film that differently. You need to focus on the terror Galactus is causing and highlight the ramifications of him destroying a planet. See the opening to the Transformers cartoon film for a good example of setting those stakes early and well. Show us refugees of other worlds who lost everything to Galactus. Show people who see the silver light in the sky fleeing their planets in terror. Focus on that, and it will feel epic.