I don't think that there is much best practice available regarding 3party DAM systems. Because it depends a lot on the usecase you need to support.
My ideal case the DAM would allow to browse the assets and let an AEM user integrate them into a page (using a link to a public facing instance). Also the DAM would be notified of any of such links, so it can keep track of them and make sure that the asset is not removed without letting AEM know.
Then there comes the asset tracking aspect (how often is the asset downloaded/displayed) and feeding that back to the DAM. And so on.
And there are probably a lot of other points which can be worth to consider when thinking about such an integration.