This is not what you seem to want to hear but in cussing, you don't have to be bald
at all to be called ハゲ. Take it from the hairy native speaker.
ハゲ - Wikipedia
Read this part on that page if you actually read Japanese:
「また不良少年同士の口論などにおいて用いられる場合 など、対象となる者の
頭髪の有無にかかわらず用いられる場合もある」