Hi. Has anyone extended volto-form-block to redirect to a Thank You page like EasyForm?
no but we had a request from a client for it.
It's not clear the best way to do this since you probably don't want the thank you page to be a regular page that can be found via search (the redirect to another page solution you mention).
EasyForm solved this by letting you edit content that goes into a view that isn't shown until you submit.
However this solution doesn't really work for form-block because since form block can't get rid of other content on the page like instructions so the end result would be confusing.
- Best I could think of is some kind of popup window with custom content and then a button that takes you to a page of the editors choice?
- Or is there some way block could replace the whole page content?
- and even it could, the editing experience is still going to be weird because it won't look like how the final page would look...
Thanks @djay. I hadn't thought about the thank you page being visible in the search. How about an Exclude from Search setting 
I think "exclude from navigation" doesn't exclude from search. So that would be inventing another setting?
One could make a copy of the Page content type just for the thank you page and exclude it from search.
We avoiding having form content types by using a block only to have a thank you page content hanging around the add menu just for an even more niche use case? this makes this less preferred as a solution IMO
@mikemets solution is nice in that I could see other uses. An exclude from search could extend to no-index metadata tag as well which I think is something that would be nice to be in core plone anyway?
You could have a button in the form-block create you the the thankyou page with those settings already but can you click a button in the form-block settings that will save the current page and take you to the edit page of the thank you page to make the editor experience easier?
But it still means you can't have secret information on this page only revealed once a form is filled out. The thank you page has a url so can be gone to directly.
But I still think the better answer is if the editor can edit the thank you page directly into the form-block so everything is self contained. Is there not a way for a block to have some sort of overlay that takes up the whole of the content area? The user to click a button to edit thank you (in form-block settings) which opens the overlay, which is a container and they put their content in there. It's all stored in the form-block and when it runs it appears as if it was a new page but isn't. possible?