Esse é um recurso do dnsmasq que não existe no unbound neste momento. Se você quiser usar isso, você precisa ficar com DNS Forwarder.
Posso configurar o Unbound como encaminhador, mas sempre enviar uma consulta para todos os servidores DNS upstream que eu configuro e receber como resposta válida a primeira a chegar?
Esse recurso existe no nsmasq pfSense:
In pfSense 1.2.3 and later this behavior has been changed to query all DNS servers at once, and the only the first response received is used and cached. This results in much faster DNS service, and can help smooth over problems that stem from DNS servers which are intermittently slow or have high latency, especially in Multi-WAN environments.
Query DNS servers sequentially By default in pfSense 1.2.3 and later, pfSense queries all DNS servers simultaneously and uses the fastest result. This isn’t always desriable, especially if you use OpenDNS and have site restrictions that could by bypassed by using a faster but less strict DNS server, or it could get results from a public DNS server over a private DNS server on the other end of a VPN. Checking this option goes back to the old behavior where it queries each DNS server in sequence from the top down, and waits for a timeout before moving on to the next DNS server in the list.