Another DCC of my acquaintance has a nice option to perform 'spline booleans', ie: take multiple closed splines (curves) on the same plane and effectively boolean them into a combined output.

I'm struggling to do the same in H. The simplest of examples is attached.

Change the boolean 'Output geometry' setting. Intersect, subtract and shatter output what I would expect. Union does not.
I would expect Union to output one overall curve with no internal part.

What am I missing something? Or approaching this the wrong way?

You can apply divide node with "remove shared edges"

Thanks Aizatulin - that fixes the issue. Do you think 'Union' should be able to solve this internally without the need for the divide node? In other words do you think this is something that should be reported?

I don't think that is necessary, since this a simple workaround (just my opinion).