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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.
Less of a note on scale and more of a note on websocket tools/abstraction - I was building something similar for an internal tool at work (locally hosted on our server, not on AWS, but very high bandwidth use for client programs). In my application for it, there were a lot of intermediate steps need for Socket.io in comparison to SocketCluster. Additionally, SocketCluster was a little more efficient hardware-wise. This link is a little outdated, but I found similar results when testing myself. That being said, as always, use what works for you!