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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'm shit at coding so everything is relatively entry level. Most give you a user interface added directly to AO3 and for the ones that do require modifying the code the creator left baked in directions on how to modify it. It's easiest to use on desktop, but I got things to work on Android by following the directions here. I created my own list of extensions rather than using the one linked in the site so I could use AO3 Enhancements for the word count features/read time estimates. If you use apple you can probably do some general googling for "install tampermonkey iOS". Keep in mind that settings aren't saved across multiple devices, so if you switch between multiple you have to individually update each device every time you block a tag or flag something as read.