I'm using Stubby for DNS-over-TLS. I would like to set it up so that it load-balances requests over ControlD's IPv4 and IPv6 resolvers, and, in case those resolvers are unavailable, fall back to using Quad9's resolvers. I haven't figured out a way to set this up. If I list all of ControlD's and Quad9's resolvers, Stubby load-balances requests over both providers' resolvers. The only way around this that I've figured out is to disable round_robin_upstreams, but this disables load balancing altogether and treats the listed resolvers as an ordered list by using the first listed resolver until it's unavailable, then moving on to the next. This slows down address lookups significantly.
What I'd like to do is tell Stubby to load balance over only ControlD's resolvers, and, if they're not available, use Quad9's resolvers as a backup. Is this at all possible with Stubby or any other method of using DoT?