I succeeded in getting a headless blog running within my NextJS site. All is well. However, when I send a post to my subscribers, it uses the ghost.io domain for the link to the post, which doesn’t have the styles I created on my site.
You may try to connect a subdomain (like blog.mydomain.com) to your Ghost Pro site, then with some redirect rules, you can redirect post links to your actual nextjs site.
Ghost mentions this case in the docs as well, but point out that the newsletter unsubscribe links would break with redirects. So, a plain simple redirect for everything is not feasible.
To expand on @muratcorlu’s answer, I think there are two options here:
Stick Cloudflare or similar in front of Ghost pro, and add pagerules to redirect the traffic you want into the nextjs site. (Being careful to follow the doc @jannis wrote while I was typing this.)
Create a Ghost theme that’s basically just a bunch of javascript-based redirects. This should not break unsubscribes, etc, which are handled before the user gets to the final (theme-generated) page.
I can confirm that Private Site mode breaks all the links in the email newsletter, so that’s not a great solution. I’m back to @Cathy_Sarisky’s suggestion of a new Ghost theme for the .io side and some redirects for posts and pages.