Okay, so the trouble was I was including the Root Path in the url when passing in the full path, and my misunderstanding of that reclassification ID.

So formatting as

https://dev.azure.com/{myorg}/{myproject}/_apis/wit/classificationnodes/iterations/sub-node/nodeIWantToDelete?$reclassifyId={ID of Root node}&api-version=7.1

Then it works. The Root Node is assumed.. so I kept getting errors not finding the path i was putting in because I had put in the path...