I made an open source DoT/DoH app for iOS called PrivateDNS (more lists and turning off on wifi coming soon just submitted for review) that includes Quad9. However from India I get very high latencies accessing some of the DNS (needed for testing my app) especially the adblocking one. NextDNS is good since it has local servers. But otherwise pretty much Google and Cloudflare is the only option that works well with Cloudflare sometimes flaky. At home I can have PiHole + unbound but I would like to have a decent fast adblocking dns while on mobile data (whenever I am outside anyway these days) because wireguard is really high latency for me and my home internet is worse than mobile sometimes.