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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.
Wagtail anyone? https://wagtail.io/
Personally would’ve gone for Ghost. It’s an open-source headless CMS that is used by companies like NASA, Apple and Sky News. There’s an article here that compares it to WordPress, and why it is better. But again this entirely depends on what fits your needs best, and if switching to Gatsby, an SSG, from WordPress, a CMS, is for you, then sure.
I used https://github.com/TryGhost/gatsby-starter-ghost, which isn't quite a theme, but comes with a decent amount of complexity, and some bugs.