Yumeship is a term made by English speakers. It comes from the Japanese words yumejoshi (dream girl) and yumedanshi (dream boy).
Selfshipper and yumejoshi/yumedanshi do mean slightly different things. A selfshipper is someone who ships themself/a self insert with a fictional character. Yumejoshi/yumedanshi is for self inserts and original characters.
Many of the people who use yumeship don't know the terms yumejoshi/yumedanshi, which leads to the spread of misinformation and confusion. (I’ve seen this a lot on TikTok)
I recommend using selfshipper, yumejoshi/yumedanshi/yume if you feel like that label suits you better, and avoid using yumeshipper!
[I am not Japanese, just saying this as someone who’s been around the selfship community since 2023 and an Asian who doesn’t like misusing words. If you are Japanese, PLEASE feel free to correct me!]