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

Questions
1

I use Safari and iCloud Private Relay.

I have Quad9 DNS to handle everything outside of Safari.

Quad9 blocks malicious URLs, but since links would use Safari, Quad9 is overridden by Private Relay anyways.

Should I leave it on? Would disabling it simplify my network and reduce edge-case failures?

2

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.