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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.
Diagrams.net (formerly draw.io) is an infinite-canvas diagramming tool that is web-based but you have control of the source files. You might also like Mermaid which is a markdown-like language that compiles to all kinds of charts and diagrams. I find generating diagrams to be a much more scalable way rather than trying to move everything around by hand so it all fits and looks nice. At some point you will want to squeeze something in the middle and it'll be a nightmare.
have you checked https://structurizr.com ?