I admit that I am not a regular user of Scribus but there are multiple ways of achieving your goals - without waiting for developers to meet your requirements.
Look for workarounds.
I do not know if you have yet explored the Scripter plugins.
From the description you give, personally I would create a template for multiple (unlinked) text frames and accompanying image frames and inject the text to be alongside images using ScribusGenerator.py
%VAR_text01% %VAR_image01%
%VAR_text02% %VAR_image02%
...
%VAR_text99% %VAR_image99%
all in a CSV fle.
In fact I extended this workaround to create *.sla via PHP include.
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Another way is to create your content in markdown editor and import the PDF.
Explore Atom editor > markdown-preview-enhanced package for multiple print options and import into Scribus. But I understand that you have an existing 500 page document so you might find it easier to disassemble document to its constituent parts (text, images) then rebuild as assets used by Scribus. Pandoc is good for this. All by command line.
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Global find and replace .. I sometimes use XMLCopyEditor on the *.sla code (which is xml).
Or apply ripgrep to the *.sla code.
https://learnbyexample.github.io/substitution-with-ripgrep/-----------------------
Remember that Scribus is a layout engine and it is often convenient to build the assets outside Scribus then import.
That is keep content and layout separate. Scribus StoryEditor tries to do both but my preference is to maximise use of external tools in a toolchain. Atom is good for this workflow.