I want to create animated, shifting geometries and vector fields, which I think Houdini would be quite capable of in many respects, but I am familiar with Grasshopper (and think it may be superior in some ways even if less so in others), so there would be a lot less learning involved.

You could try IOGRAM (iogram.ca). Some demos here....

Wow, thanks Daniel, glad I posted my question, I will try it out,
looks very exciting.