Have you considered using the from integration with your CMS?
That or if you have the resources you can try and API:
Great to hear you are getting off the iframe. The become hard to scale as an option.
Thanks so much! These are great suggestions, I'll look them up.
Would gladly hear what others are doing in order to get rid of the iframes as well
ALE does this on our public site which runs on Sitecore. For example look at https://www.al-enterprise.com/en/contact-us. The HTML originates from the form built in Eloqua. Then we edit the HTML to change the styling so it is using stylesheet elements from the site so it looks nice. Our Sitecore code puts a hidden field in the form called ConfirmationURL and we set the return page to the website page we want in Sitecore. You could of course do this manually in the HTML once you put it in your site too. Then the form processing it set to go to an external URL based on the value from that form field. So in the end a visitor submits the form to Eloqua which captures the data & then redirects back to our website.