I don’t think this accomplishes what he wants. The router DHCP will assign the second DNS address as you mention, but the devices will select one at random, not as a backup/failover. So what happens is that devices sometimes go through the Pi-hole and sometimes go through the secondary DNS address and receive ads. The only real way I’m aware of is to have a second pi-hole for redundancy. Personally, I decided to use a cloud based service (NextDNS) for this exact reason. I didn’t want my families internet to rely on devices that I host.
I’m not exactly sure how many queries, but it’s above the free limit. I purchased the pro plan. For $20 a year and it’s been a great service for me. I can send a referral code for 30% off (I think). I think adguard has a similar service.