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webiny-js
Open-source serverless enterprise CMS. Includes a headless CMS, page builder, form builder, and file manager. Easy to customize and expand. Deploys to AWS.
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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
Webiny 100% hits all of those points you're looking for. It's built for organizations that like to create something amazing and different with it, therefore you can manage everything via API, create plugins to modify the UI or core functionality like GraphQL resolvers, and it has a WYSIWYG page builder. Check it out and send me a DM if you'd like to know more. https://www.webiny.com