Make sure you import the .targets file that defines the MonoGameContentReference build action in the .shproj file by adding the following line:
<Import Project="$(MSBuildExtensionsPath)\MonoGame\v3.0\MonoGame.Content.Builder.targets" />
But it’s true that it’s a bit of a hassle setting up a shared project manually like this. Once you get it working it’s real nice for development though 