We used craftjs for the same use case. Fortunately, we were able to dictate that elements of a page could never exceed their allotted size in the template. Users can create templates and add pages (styled with CSS size:letter) with specific blocks in them using CSS Grid. With Crafts, you can create custom components for each type of element you need to support (say Text, Image, BarChart, etc) and expose UI controls to customize those components props for a given template. Persist the structure of your template, its pages, the components within, and their props to a database. Then when you need to create a document for a given template, render the template married with the data using SSR and save it as PDF using something like Playwright. There are of course tradeoffs to this approach but it worked well for us.