Maybe take a look at profiles https://github.com/duct-framework/duct/wiki/Configuration#duct-base-profiles-and-modules

I don't think the wiki mentions it, but you can create your own profiles

The profiles are a bit inverted in terms of control. I have a core library and project that uses it. I would like to have a partial config in the library and then in the project I;’d like to import that config and fill in some gaps… I don’t know if profiles are great for this… I guess I can always manually merge edns

Profiles allow you to merge any edn resources into your:duct/core config. Sounds like that's what you need but maybe I'm misunderstanding

I want the library’s version be the base and then the web server config to be an override, but using profiles, that would make the web server config a profile I must always enable?

Yeah that sounds like an edge case. Not sure how you'd resolve that