Attempt 1:
Using TextureImage.open(manager, primaryTex) and TextureImage.open(manager, secondaryTex)...
ResourceLocation primaryTex = new ResourceLocation("textures/item/" + primaryId.getPath() + ".png");
ResourceLocation secondaryTex = new ResourceLocation(MY_MOD_ID, "textures/item/my_mod_underlay.png");
ERROR:
java.io.IOException: Failed to open texture at location minecraft:textures/item/netherite_boots: no such file