@everysummertime Having two blocking mechanisms in parallel, but only some of the blocked connections showing up in Little Snitch caused quite a bit of confusion. It could happen that a connection was silently blocked via DNS, and users were wondering "why can’t I connect to that server, although LS doesn’t show this connection as blocked".

That said, if you really want to let Quad9 also filter your connections, you can create a custom DNS encryption setting that uses this filtering service.