Well, Caddy isn’t able to resolve your host to an IP, because it’s using the default system reoslvers to try to do so.
I think you need to also configure resolvers also inside of transport http. There’s resolver config for SRV (which is to do the SRV lookup itself) and resolver config for the transport (which is to do the actual HTTP connection upstream, including resolving the name to an A record).
Add this in your reverse_proxy config:
transport http {
resolvers 127.0.0.1:1053
}