It will not help to improve the state-of-the-art wrt supported features, but in some testing I did a few months ago it turned out that you can use just about anything that Assimp supports for meshes in urdfs. In the end it all gets converted to vertices & faces anyway, so as long as Assimp can load your mesh it should work.
IIRC I even used fbx at some point.
Note that this is really just about meshes, so none of the ‘advanced’ features formats like Collada and FBX support are used (bones, skins, kinematics, simulation properties, etc).
I did not include Gazebo in these tests, so that may be different.
Collada has a variant where the whole file is compressed using gzip, the extension changes to .zae and it typically results in 90% reduction in file size. It supports all of the things you mentioned, but it is definitely not meant to be human readable and is not straightforward to export to without a library doing it for you.