The code in my Pen is deliberately compatible with IE 11, so if you change it just to use late-model JS module import and export, I won’t be able to help you with it. Preserving compatibility is important to the wider community.
In any case, it’s unclear what you mean by “way too fast.“ Either the form submits or it doesn’t, and you can check this in the F12 Network tab. The Thank You URL is only known after the form is successfully submitted (assuming you mean the TY page configured in form editor), so there’s no way for the form to skip to that URL without first submitting.
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