@Ben - quiet your end.

I have mentioned MarkDown several times.

The product that is near perfect for your needs is Obsidian.
It is multi-platform/OS but all local storage, hence private.

All your notes would be in MarkDown and are editable, searchable, and copies are saved and stored under your control.
No overhead and external image files are just linked in.

It comes with Core Plugins for Search and Templates.
Community Plugins for LaTeX, Longform for book management.
the Pandoc Plugin allows you to export documents to Word Documents, PDFs, ePub books, HTML, PowerPoint and LaTeX among (many) others
It's so good, I would use it myself. Oh! I do.
It's free. You can pay for Sync & Publish, each @$8/m.

For you, my doubt is your older OSs.

Acorn