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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.
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Superforms
Superforms is a SvelteKit library that helps you with server-side validation and client-side display of forms.
As for exposing the API externally, neither (can) limit this but SvelteKit's API is generally considered to be an internal implementation detail that you don't use directly since it might change between versions. If you want to expose an API you should choose tRPC, probably alongside the OpenAPI plugin or something similar.
My suggestion for self-contained apps is to use the standard load functions until they're not enough, and then add tRPC. trpc-sveltekit provides an excellent integration.
I personally think SvelteKit's type-safety + something like https://superforms.vercel.app/ (or just plain zod if you prefer) is perfect.