I notice you get quite a few questions on mixed content which happen after someone moves a site or move it from http:// to https://
I did this now a few times and notice the following which might shortcut a lot of problems/questions, maybe you like to take it up as an issue in the documentation.
When you move a site, you export&import the GP settings. You also export the sites with content including all media files.
The problem is mainly that the URLs for the images are not getting updated for the new site, but are still referring to the original site, i. e. if the original site had no SSL, you get a mixed content. If it had, you don´t notice this, until you delete the original site and your images are not shown anymore… Another problem is in cases where the original site had an image set in the Customizer which is now deleted at the new adress, but the URL still exists in the code. So you do not have the image as media file, you do not see it anywhere, but you still have a mixed content because of the URL in the code.
As for now, I found the fastest and easiest way, if one does not have hundreds of images. Simply delete the image and add again in the Customizer.
In case you get a referring to an image, you do not use anymore, simply add ANY image in the Customizer to the place where you had the image before, save the site and then delete the image. Save the site now and it removes the code without the need to search further for mixed contents. I hope it is understandable…
I know you suggested in some cases a plugin that updates the URLs. But maybe there can be a nices solution later, where GP updates the image URLs when the site is moved. 🙂 Actually I think it should be a WP issue to be solved.
I am quite new to WP and GP and this might not be the nicest solution, but it worked for me the best and might help others, too to have a fast remedy to mixed contents.