So you've talked about how there's like..... a Goal, but they don't discover it until later.
Why?
This isn't me asking the question, but it's a question to make sure you can answer yourself. Why not start the story where those goals come in? You have the power to start the story at the point where the Big Bad has gathered enough power to be a threat, or the point when those goals are revealed!!
Because, like, this is very reductive, but if someone asks "how do I make the story interesting while my characters are still faffing about?" then the answers is, that's the wrong question -- the real question is, is there a story there at all? If they don't have anything to do other than mindlessly punch monsters, is there a reason you don't want to skip ahead to the interesting stuff? TO BE CLEAR: you might find that you DO have a good reason for those chapters! But you gotta make sure there is one other than "I always imagined this other part would happen later."
Also -- I think conflict might be an important part of motivation, here. Like, I assume they have a Reason™ to fight bad guys, but like........ is there any reason NOT to go fight bad guys? Because if your character wants to go punch a bad guy, and has no reason to not do it, and goes and punches a bad guy, then there's kind of, nothing in his way? And that makes whatever motivations he might have feel sort of hollow.
There's a lot of different directions to take this that introduce a potential reason not to punch - needing a Special Weapon That's Hard To Get, or needing to keep the mission secret from someone, or being afraid -- but I think ultimately a lot of this stuff can't be tacked on for a chapter; it has to be built into who your characters are. The character who wants to be a hero but isn't strong enough, or the character who is powerful but fears his own destructive rage -- these people might discover some bigger goal down the line, but they already have a conflict inside them. And there's a good chance that these smaller conflicts will tie into how they approach the bigger conflict, which makes them a lot more relevant to the story overall. It's not just "how do I motivate them for this chapter," but, "how do I use this chapter to start to reveal their deeper motivations?"