PDFs are inherently not designed to be edited, the format lacks a lot of the information necessary for layouts to work correctly and as expected.

That's why you have to open it with LibreOffice Draw, and the mess you see is basically the information that's contained within the PDF. It is just a bunch of random text cells randomly placed over the page. That makes it really difficult to get back an editable version that's sensible. Page wraps and such will never work correctly. Your only chance at recovering it is if you can figure out what software wrote it, and how different constructs might end up when translated to PDF and a lot of heuristics.

I believe they open a bit better in Xournal++ but it still sucks.

Those that do build such tools realize it's all big companies with big budgets that really have a serious need to do this, so they tend to be proprietary and expensive, and still not super great.

I would really beg for the files to be provided in a suitable format for editing.