Experience Fragment - With a web form - Different Thank your page across different sites

Hello All,

This may be an easy solve or an attempt to use the wrong solution, so please correct us if you know of a better way! Also, we are not using any of the advanced Experience Fragment features like building blocks or variations in this case. 

Our quandary: We have chosen to use an Experience Fragment as an author controlled means to replicate a Web Form across multiple sites. Everything works well, except we can't seem to configure a site specific form submission "Thank You" page to work across each use of the XF. We *tried* setting the forms "Thank You" property to a generic relative path called, for example:  "ThankYou.html" and putting that page in the same folder as the page the EX form exists in each site. That unfortunately fails. It looks like under the hood the XF Form attempts to resolve the path for the ThankYou page and that fails because we didn't provide it a valid path.

Setup Outline and Desired Outcome:

Experience Fragment: Fully built form which has a Thank You page configured on submission.
Subsite 1: Page containing  the EF Form previously mentioned, In the same folder, Site specific Thank You page.
Subsite 2: Page containing  the EF Form previously mentioned. In the same folder, Site specific Thank You page.
Subsite 3: Page containing  the EF Form previously mentioned. In the same folder, Site specific Thank You page.

Desired Functionality: On submission of the page containing the form we would like to take the user to the site specific thank you page. 

Current Problem: Can't seem to configure an Experience Fragment with a site specific thank you page url as the form submission attempts to resolve the configured property and expects, I assume, a qualified path, not a pagename with no path like we are trying to use.

Is there a better or different way to solve this problem in an author-centric way? 

Many Thanks,

Tom.