To answer your question:
Can you explain why dirmngr does the DNS roundtrip lookup, mapping from the pool's A and AAAA addresses back to names? It seems like it'd be a lot simpler (and faster, and less error-prone) to avoid the PTR lookups if we have the IP addresses already.
If it is a plain server and not a pool, looking up the PTR is
necessary to get the hostname for SNI and possible also for Host:
header.
For a pool we would not need the name because the already known name
of the pool is used for SNI. However, to find duplicate hosts in the
hosttable it is useful to have the hostname. We also return the
actual used hostname to gpg for information purposes and to eventually
store this with the key as meta info.
Yes, we could the PTR lookup of pools faster or in the background -
but for now a simple appraoch is better for debugging.