Currently, creating queries for custom data is well documented, which is cool and definitely above average for an opensource project, thanks for that!

The process is still quite long however, and could IMO be supported by e.g. using macros (my knowledge of kotlin metaprogramming is low :-(), e.g.

@CustomContactData(ZeroOrOne)
class WhatsAppCallContact(): CustomContactTable {
  val mimeType = MimeType.Custom("vnd.android.cursor.item/vnd.com.whatsapp.video.call")
}

For the gender case, I'd imagine

@CustomContactData(ZeroOrOne)
class Gender(): CustomContactTable {
      override val isBlank: Boolean
        get() = propertiesAreAllNullOrBlank(type)
  // I don't know if Kotlin supports this
  type Type = CustomGenderType
}

or as an alternative, an android-studio IDE plugin would also work. I think there's even a tmbundle format for snippets, which a lot of editors can use.