Is your setup actually likely to run into situations where the DNS server is unavailable but Internet access is otherwise working? Basic home setups usually have DNS running (if at all) on the same device (router) that’s also acting as a gateway, so such a selective outage would be unlikely except in the event of misconfiguration - and in that case I’d want my setup to fail hard, personally, so I’d know to fix it, rather than having it silently slow down DNS lookups. Thus I wouldn’t intentionally set a public DNS server as an alternative to the primary one on the router. But maybe your setup is more complex, or you have a use case I’m not thinking of...