Can't you tell the user to swap out the schema when needing intellisense for other than Umbraco? So you basically have more than on

$schema

in your json, but comment out "inactive" ones? like

{
  "$schema": "https://json.schemastore.org/appsettings.json",
//  "$schema": "https://usync.org/appsettings.json",
//  "$schema": "https://doctypegrideditor.skttl.dk/appsettings.json",