Do I need Quad9 DNS if I use iCloud Private Relay?

Private relay uses ODoH by default so your DNS requests are protected by the same dual hop architecture as the main service. For blocking ads and malicious URLs systemwide, Apple released the URL-filtering API which handles systemwide ad blocking in a privacy-preserving way. The first app that supports this API released recently, if you buy Wipr 2 and enable Filtr, you’ll get the privacy-preserving systemwide ad blocking. I think leaving the DNS default with private relay and using Filtr is the best move here, but it’s quite new and might have some quirks to iron out. I think AdGuard are also working on implementing this API so you could wait for them if you want to.