It would be great if you could add compatibility with the Aether mod's (https://www.curseforge.com/minecraft/mc-mods/aether) Valkyrie entities so that they would be able to be traded with for custom Aether villages.

Did you already tried to use the "Custom Models" feature which allows you to overwrite the model with any compatible entity?
See: https://github.com/MarkusBordihn/BOs-Easy-NPC/wiki/CustomModels

If yes and this is not working, it would be great if you can provide me a specific example to reproduce. Thanks.

I just tried it out. The model itself loads but it has no animation. It stays static when moving, attacking, or being hit. I'm not sure what you mean by specific example, but I just used this command if it helps: /easy_npc render set entity 62c4c1de-ac1b-4ddc-92f4-af44b0bc2d8c "aether:valkyrie"