Not a user script, but the LibRedirect extension does most of that already https://libredirect.manerakai.com/ https://libredirect.manerakai.com/ I think it do

Not a user script, but the LibRedirect extension does most of that already https://libredirect.manerakai.com/ https://libredirect.manerakai.com/ I think it doesn't check whether the chosen instance is up before redirecting, but switching to another one if that one is down is a single click.