Elves in Worldbuilding

I've done the majority of my essential worldbuilding for a particular campaign world I'm working on but I'm struggling with my elves-specifically a particular portion of them. Races are not homogeneous communities in my world and have plenty of cultural, geographical and societal divides. But there is a country of elves in the centre of the map that is considered by them to have sacred significance as it's where they believe they entered the material plane. Historically speaking they have been at war with a nearby empire and several other nations, including dwarves and tieflings, and still hold some prejudice against dark elves but not outrage hatred. Due to this violence they are quite isolationist. For extra background my world is vaguely approximate to early 19th century with some even more modern technology and infrastructure.

I still need to build a culture surrounding these elves and this place. Do people have any advice for when you get completely stumped on building a culture? A spark of where to start or a resource to pull from? This includes traditions, architecture, language, arts and crafts. (I don't want to make them a stereotype of a real world race/nation and I don't want to do the usual tolkien elves).