The theories for Azhdaha being the Geo sovereign mostly rely on Zhongli calling him old asf and some of the language for him (Dragon Lord, Lord of Vishaps) also being used for sovereigns (not exclusively though - Beidou's title Dragon Lord uses the same characters). as far as i know Dvalin has no comparable evidence and Kapatcir is just a big bird. And yes they are more related to vishaps than whatever Zhongli is, the research facility in Enkanomiya implies that vishaps are just young and/or unevolved dragons.