Zoro's Dream is about Power
Usopp's Dream is about Bravery
Robin's Dream is about Knowledge
Nami's Dream is about Exploration
Franky's Dream is about Invention
... etc
Zoro can Train, Usopp can become Braver, Robin can Study, Franky can Invent, Nami can Draw her Maps ... etc
But the reason these Dreams are considered "Impossible" is because of Obstacles that They need to Remove,
As long as SHs don't "Remove" these Obstacles, They can't say that They have achieved their "Dream".
Zoro's Obstacle is Mihawk
Robin's Obstacle is World Government (Celestial Dragons at least)
Sanji's Obstacle is Red Line
Usopp's Obstacle is Elbaph
... etc
It's like SHs need to "Overthrow" Others who already have a "Bad" Version of what They are looking for.
Mihawk is already Greatest Swordsman, Elbaph Giants are already known as Brave Warriors, World Goverment already know what happened in Void Century (At least Imu & Few others), Red Line already connects Four Blues through Reverse Mountain, Current World is already Mapped but it's not as it should be, even Chopper's Dream already exists in the Form of Devil Fruits ... etc
SHs Dreams are achieved when They "Re-Define" or should i say "Re-Create" those Treasures, not when They copy what's already there.
In other words, They won't just Discover Stuff, They will Manipulate The World
Zoro won't just Discover Peak Swordsmanship (Mihawk's), He will Create a New Peak
Robin won't just Discover what happened in Void Century, She will Re-Write it
Nami won't just Discover World Map, She will Re-Design The World herself
Sanji won't just Discover All Blue, He will Create One
Jinbe won't just Discover how to Unite Races (Fishmen Mainly), He will make Them One and the Same
Chopper won't just Discover All Healing Cure, He will Create it (Probably involves turning DF Cursed Tree into a Sakura Blessed Tree)
... etc
So No, Usopp didn't achieve his Dream, not even Close.
His Dream will be Accomplished when He establishes a New Elbaph Order & becomes their Symbol of Bravery.