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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.
Either way, I'm not too familiar with RxDB but looking at the docs you can also use StepZen to create the GraphQL layer that you use for replication. With StepZen you can build and deploy a GraphQL API based on multiple datasources (including MSSQL) in minutes. That way you only have to worry about building the logic that pulls and push replicates data to and from GraphQL. How you structure the MSSQL database would become the most important part of this project, to ensure a smooth replication.