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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.
I suppose it depends on what you mean as a growing app. A few of the examples (https://github.com/yolmio/boost/tree/main/examples) are apps that you could easily sell as a freelancer/contract work.
You can of course split up your system into as many files as possible, the emphasis on a single file is that now instead of your architecture, languages, etc. enforcing separate files and hampering reuse, you can organize it however you like.
I'm sure best practices will evolve overtime, but I've been alright with just a single file for apps in production (as long as the majority of components are made reusable)