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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.
If you need perfectly flat lines while you edit then starting from https://github.com/fenollp/reMarkable-tools and implementing this kind of heuristics could help.
If you are really into this kind of tools then https://github.com/kenperlin/chalktalk is pretty amazing but it's not for the reMarkable, more for something like an iPad plus, just like the kind of heuristic I suggest, makes for a tool that requires a bit of training from the user to (ironically enough) feel natural, in my experience at least.