Clean to Grid filter did the job! Thanks! So the pipeline was my unstructured grid volume → Clean to Grid → Extract to Surface → Contour.
Last question, are contour labels supported yet? I did a fair bit of googling and it sounds like it might be a feature request that has been pushed back for several years? I did find a post here talking about it that might have a temporary solution (Is it possible to add labels to contour lines? - #15 by wascott), I will attempt that and see what I get.
Thanks again!