At work I selected GraphCMS to be our preferred headless CMS. It is already leveraged on multiple projects and has been working out well for us. The beauty of a good headless CMS is you can use in many scenarios. Case in point, of the 3 instances we have, 2 are web apps; only one is marketing site.
If you are not familiar with GraphCMS, it is a flexible headless CMS with native GraphQL support.