So then Ben, I wish you well with Affinity.
You originally specified "File size is important then" with "all the graphic files need to be externally linked".

Your 737 MAX file with only text is 132kB and its PDF is 415.42kB for all of two and a half pages.
Affinity offers little in the way of embedding external content.
The switching back and forth with Affinity is on a page-by-page basis.

You also said you were "an early adopter of Windows 10" and getting W11 but here you are now asking about W7.

Xara Pro+ will do what you want with a first page preview image overhead that I have reduced dramatically for you. Xara could probably eliminate this totally if you advised them of the issue.
Xara comes with the joy of in-line graphic and vector manipulation alongside text.
The trade-off is around 10kB per page of text that is a factor of three above raw ASCII.

Both Affinity and Xara use proprietary file structures so if the product breaks or disappears so does your archive or book.

On the other hand, MarkDown is plain ASCII that is parity with plain text.
It is human-readble and there are many applications that will handle .md files.
External image linking is trivial and goes where you put it.

Your only loss is in-line vector creation.
If I did not have XPro+, I would exclusively use Boxy SVG as I can deliver SVG animations and retain the same SVG for re-editing.
The SVG code will paste directly although is is better to use an image link; Obsidian allows drag and drop into any document file as will any other good MarkDown editor.

I will not trivialise the task you have set yourself but you are juggling with system and archive instability without addressing your computers first and then how to handle all the metadata around achieving a fully integrated and searchable archive.

For almost in-line LaTeX in an .md file you could use add a <script> line: <script src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"> </script>and thereafter surround LaTeX with $$: $$V_{sphere} = \frac{4}{3}\pi r^3$$. This also works in XPro+ but the text needs to be turned into a Named htmlblocktext area.

Obsidian + Boxy SVG >> Xara Designer Pro >> Affinity Suite.
I think you would be better with a purchase (https://www.xara.com/webdesigner/v20/) rather than a subscription.

Acorn

P.S. I would have liked some sort of acknowledgement for the effort put into Post#17.