There are many options for a headless CMS and a lot of these options have a free tier. Manage your content in one system and consume it with the front-end framework of your choice.
What problems does Headless WordPress solve?
By using WordPress as a headless CMS, you get the best of both worlds:
A world-class backend system combined with a modern frontend.
A headless architecture is a decoupling of the content creation and the content presentation layers of web based content. There are a lot of paid solutions available (search for headless CMS if you don't believe me!) but none of them are perfect.
This shouldn't be surprising to us. Most general solutions to any problem do 80% of a job well, right?
The Pareto principle in action.
But, that last 20% is the kicker.
How easy is it to achieve that custom 20% with these platforms?
For your particular use case, is there a content structure that would just make your life easier?
Or is there custom logic that would help your editors speed up their workflow?
Developers have been bending WordPress to their wills for decades. The plugin ecosystem is vibrant. There is a lively community of open source contributors sharing solutions.
As a WordPress developer, you are in charge of what your API looks like. You control how to present and structure your content. You get to help your editors as you build a robust and powerful editing experience.
You're not starting from scratch though. WordPress comes with a REST API when you start a new site. If you prefer GraphQL, then the wpgraphql plugin has you covered.
This book will show you exactly how you can build your own headless CMS with WordPress.