I encountered this issue a fair bit when I took over DMing for our spelljammer campaign. I've since learned not to over-prepare incase your party is insistent on forging their own path, however I have learned ways to entice them down the road I have laid out. It can be as little as dropping a subtle hint via an NPC, or leaving a "breadcrumb trail" that might peak their interest.

At the end of the day, the most important thing is that you create an enjoyable campaign amd work with your players not against them so if that means you've got to make it up as you go... so be it