I think it's relatively new too.
You mentioned Strapi, maybe sanity.io/?
Look promising, but never tried it personally.
I wanted to get started(properly) with something based on firebase.
Part of the challenge is finding one of these that will work well for clients. If I need to update something, none of these would be a problem. I feel that the layout and maybe some of the terminology could be problematic for a client who isn't super tech-savvy. That is one big benefit to Wordpress is predefined fields. On my to-do list is trying Wordpress as a headless CMS using the rest api they offer.
Just remembered I had something else contentful.com/.
I'll leave it to you to figure them out, it's a little too much for me :p
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