Personally I have found Google to work well almost always. The reason I like DNS on iPhone itself is because assuming the iPhone is super optimised for handling https traffic and also an extremely fast device, not to mention a very stable software platform compared to the terrible home networking gear people own that has some outdated linux firmware that can be buggy at dns resolution. For most on HN its probably not the case but I've seen too many people use old hardware for a basic home internet setup and I'm hoping using on device DNS can greatly improve the experience. At the cost of small hit to the battery life of course but I would have to test that which would be a fun test to do.

Have you sent a traceroute and the results of a chaos query to your ISP, so they can fix whatever routing problem you're encountering? There's nowhere in the continental US that's more than 5-6ms away from a Quad9 server cluster.