You can now turn off autoUpdate in ServerNPC's skin.yml file. To avoid skins refreshing every 5 minutes, simply disable it in the skin's config. This stops ServerNPC from redownloading that particular skin.
Example of disabled skin autoUpdate:
ServerNPC/skins/mojang-hello.yml
Code (Text):
skin:
==: com.isnakebuzz.npcapi.skins.SkinData
value: 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
signature: 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
uuid: '2ee745d2-8d21-4255-9b61-c50486592b95'
name: hello
timestamp: 1692395060881
autoUpdate: false