Syntax and semantics are never orthogonal and you always need syntax so it must be considered from the start. Any reasonable syntax will quickly become much more pleasant to generate an ast or ir than, say, manually building these objects in the host language of the compiler which is what the semantics first crowd seem to propose.
It also is only the case that most of the work is the backend for some compilers, though of course all of this depends on how backend is defined. Is backend just codegen or is it all of the analysis between parsing and codegen? If you target a high level language, which is very appropriate for one's first few compilers, the backend can be quite simple. At the simplest, no ast is even necessary and the compiler can just mechanically translate one syntax into another in a single pass.