I did have similar problems. Besides collecting emails, I wanted to use a popup to highlight a specific blog post.
Usually there are hundreds of popup plugins on the platform like wordpress but ghost does not have such a plugin unfortunately.
After researching, I found popup builder services that I can create using popup for ghost. The services I will recommend from these are as follows:
https://popupsmart.com (Free up to 10,000 page views) * the service that i use
https://sleeknote.com (49 euro per month)
https://www.hellobar.com/ (Free up to 5.000 page views)