Indeed there is no way to do a mixinto in to a js or html.
To don’t interfere with other mods when overwriting the js or html you could do a submod and do a hotload, the kind of stuff that works on selecting a kingdom but I don’t know if that can be made so it works on a selected biome. See raya’s childeren ui mod for more info.
But even If you could made work with a hotload, you still have to check every new build if the js/html/less hasn’t changed.