Then have the Thank You URL be part of the OM embed code instead. Put it in a JS variable.
I'd never clone forms just to set Thank Yous. Maintenance nightmare.
Ok. This solved it:
MktoForms2.whenReady(function(form){
var formEl = form.getFormElem()[0];
var submitEl = formEl.querySelector('button[type="submit"]');
form.onSuccess(function(vals,tyURL){
var loc = document.createElement('A');
loc.href = tyURL;
window.location.replace("https://my-special-thank-you-page");
Thanks for the coaching!