I am using Unbound and Cloudflare to add Pi-hole with address 127.0.0.1. When I restarted the device, I found that Unbound and Cloudflare were delayed about 4-5 minutes before performing DNS resolution.
Fortunately, my ISP already has IPv6 so I added Google IPv6 DNS to backup. Every time I restart the device, Pi-hole can still resolve normally without delay. But so my DNS queries can be switched to Google servers for random resolution (if query times are faster than Unbound and Cloudflare).
In the future, can the Pi-hole add the DNS fallback feature to backup every time Unbound or Cloudflare can’t boot every time the device is turned on?