Altium: Polygon Pour Direct Connect on Vias, Relief Connect on Pads
When you're setting up a new PCB rule, you can type "query text" to match objects to which the rule should apply.
When I do what you're trying to do, I just use "isVia" as the query text and then set the Connect Style to Direct Connect:



I'm not sure how to exclude your QFN's thermal stitching vias though...