Prismic's "slices" might be what you're looking for if you want out-of-the-box functionality and hosting.
If you're wanting a bit more flexibility from a code-perspective (and you don't mind TypeScript), I've found Payload CMS to be a great option to build out a page builder as you've described using their "blocks".
Looks like Prismic and Payload CMS aren't open source?
https://github.com/payloadcms/payload/blob/master/license.md - "Payload is free to download and install—and free to use for Personal Purposes. We are passionate about Payload, and we hope that you will enjoy it, too. A valid license is required to utilize Payload as detailed below"
Ahhh thanks for the shout out to Payload! I'm one of the founders. seanwilson, Payload is technically open source and completely free forever for up to one admin panel user. There are no restrictions at all besides that and the code is fully open on GitHub.
Would this model work for you, or would you like to see changes to our current model?
> Payload is technically open source and completely free forever for up to one admin panel user.
Can you explain how the licensing works here? If it's open source, couldn't you modify it to allow more admin panel users? Can you self-host with no reliance on other hosted services or paid licenses?
If you're wanting a bit more flexibility from a code-perspective (and you don't mind TypeScript), I've found Payload CMS to be a great option to build out a page builder as you've described using their "blocks".