Have you tried an open-source headless CMS? Any recommendations? (No WordPress allowed)

I haven't tried an existing one, but I built one with Phalcon in PHP. Full backend management with an API on the front-end that the client had their developer use Angular to interface with.

I have used a few, like Squidex, keystonejs, Strapi or Ponzu and can recommend Cockpit CMS. It is easy to customize or change by non-php developer, has a great support on gh issues and is simple to add features by adding plugins (like for full text search or backup). Feels mature and complete. Profit 💪