Means simply firstborn see my post 184 Colossians refers to Christ as the first born of the Father, Exodus 13:2 Sanctify unto me all the firstborn, whatsoever openeth the womb among the children of Israel, both of man and of beast: it is mine.
Means simply firstborn
Ok, using your definition does nothing to change the fact that Joseph and Mary had other children.
The key is context.
In the eight passages where this is used in the NT it does not limit the meaning to just one. It does not limit the passage to mean just one or only as monogenes does.
Again, context is your key on how a word is used.