Why not download the most popular DAV libraries from various languages, Java, C++, PHP, etc. Regardless how ancient they are.
And then have AI like Claude to analyze and bring in the improvements to your own Go library?
I was doing something like that for Kerberos and Iceberg Rest Catalog API, until I got distracted and moved on to other things.
You don't have to use it to directly write code. You can use it just for the analysis phase, not making any changes.
I think the issue is mostly that it desperately tries to avoid filling its context window, and Anthropic writes system prompts that are so long it's practically already full from the start.
A good harness to read code for you and write a report on it would certainly be interesting.
How close to retirement are you?
There were also a bunch of fun things with quirks around unicode filename handling which made me sad (that was just a matter of testing against a ton of clients).
As for CalDAV and CardDAV - as others have said, JMAP Calendars/Contacts will make building clients a lot easier eventually... but yeah. My implementation of syncing as a client now is to look for sync-collection and fall back to collecting etags to know which URLs to fetch. Either way, sync-collection ALSO gives a set of URLs and then I multi-get those in batches; meaning both the primary and fallback codepath revert to the multi-get (or even individual GETs).