Congrats on the great coin. I don't know if its possible to clean it any better than you already have.
As the copper corrodes in the ground, it forms that greenish layer called patina. Some times its very smooth and shows great detail of the coin. Sometimes its rough (like yours) and hides the detail. Regardless of which type you have, it is a terrible idea to try to remove it. Because that surface layer is corrosion, if it's removed, all you have left underneath is an ugly, pitted, unidentifiable copper disc.