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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.
You might take a look at a framework that already includes user authentication like Feathers.js. The Getting Started Guide walks you through creating a simple real-time chat app with user authentication. And the final chat app code is on GitHub here.
Keystone also comes with user authentication out of the box. It's GraphQL focused. I know Keystone v5 supports MongoDB, but not positive about v6.
You might take a look at a framework that already includes user authentication like Feathers.js. The Getting Started Guide walks you through creating a simple real-time chat app with user authentication. And the final chat app code is on GitHub here.