Have you ever worked with WordPress? If not, count your blessings. :)
But seriously, using a CMS like WordPress does provide a lot of useful features. The feature I miss most as a MERN developer is the media library browser. All image fields are edited using a modal where you can select something previously uploaded or drag-and-drop new images from your file browser. Well, now you can have something similar for your React app.
Note: this only includes the UI; everybody's app is different, so I can't write your API connections for you.
Install it via npm in your project.
npm install react-media-library --save
See Storybook.