I'm not sure about Android, but you can disable iCloud Private Relay by returning NXDOMAIN for the domains below. These are Adguard filtering rules, but you can configure something similar using dnsmasq or Pihole.

  ||mask.icloud.com^$dnsrewrite=NXDOMAIN;;
  ||mask-h2.icloud.com^$dnsrewrite=NXDOMAIN;;
  ||mask-api.icloud.com^$dnsrewrite=NXDOMAIN;;
  ||mask-t.apple-dns.net^$dnsrewrite=NXDOMAIN;;
  ||mask.apple-dns.net^$dnsrewrite=NXDOMAIN;;
  ||mask-api.fe.apple-dns.net^$dnsrewrite=NXDOMAIN;;

I use these to automatically disable iCloud private relay, and I also have rules on my edgerouter to force certain devices to use my adguard instances for port 53 traffic, and it works well.