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.
But it is not a fact that Joseph and Mary had other children.