Consider also offering Viber and Signal for interviews. Only Facebook owned things are worse than Google for privacy.
I take it you are also signed up here:
https://www.nngroup.com/articles/
I designed a lot of UI stuff from 1983 to 2012 and found NN UX articles useful. Current Windows 10, Many Websites (esp. Dark UI techniques on Amazon) and every version of Android, including AWFUL Android TV are examples of stunningly bad design ignoring every lesson learnt from 1960s to 1990s.
I'd have not have chosen Calibre as a subject as it's totally untypical. I've used it since maybe version 2.x on XP. It's not untypical of the problems of presenting a view of a database and having options for many features, often 3rd party. Also to an extent the GUI can be dramatically changed (more than one menu bar, a conventional title bar with a menu.
Also answer in your thesis: Why did MS EVER think it was a good idea to hide the LEAST used items in menus?
1) I turn that off. It's absolutely evil. Started about 2000 or 2001.
2) Frequency of use is totally unrelated to importance!
Also Calibre isn't about usage of ebooks, but managing a library of them for any number of eReading gadgets.
The actual library & sorting on actual eReaders varies from just acceptable (Kobo with Collections and Series as well as Authors and Titles) to broken (Kindle Collections and Series are broken and can't usually be managed by Calibre) to really non-existent (simply a list of all authors or titles, or worse, only a directory & file browser).
I've used and written document management and archive systems for Windows and also DOS and CP/M based book lending library and video lending library programs.
The only ereader that's even close to mid 1980s document/library features is the models with current Kobo firmware. Calibre isn't actually a library system. It manages access to books on your PC/Laptop to transfer them to a device. As such it provides obvious metadata, searches, views and ordering on those and utilities to manage the books such as covers, series, collections, metadata (for the ereader) and ability to change the format or check for structural errors, preview on screen, fix / change the system TOC/NCX or edit the style sheets and content.
It's absolutely not a traditional document management system nor suited as a library in the sense that physical libraries, video libraries or Borrowbox (virtual ebook and audiobook lending library).
It's needed because:
1) In the past an ereader didn't have space for lots of ebooks. Not such an issue now except for PDFs (which are not ebooks) and comics/graphic novels.
2) Apart from Kobo the book management software on ereaders and ereader apps is abysmal, and even Kobo isn't great. Amazon used to let users manage collections on the device or Calibre. Now it's so called Cloud based and all your devices get the same and it's broken. They recently added Series, and it only can use Amazon supplied books in the Amazon order. Broken.
3) An old pre-Amazon Kindle Sony, and certainly the ancient PRS350 and PRS650 are better at managing books than a kindle.
If you use other than Amazon's cloud (and you should) or more than 50 titles or more than one device or need to change formats, then Calibre is a necessity, because the ereaders and ereader apps (more than 1/2 ebook readers use a phone or tablet app) are dreadful.
Apple is a problem due to their walled garden.
Android gadgets using MTP rather than mass storage can be awkward.
Many Apps import ebooks, so Calibre can't delete them from the app, and the App duplicates the content you put on the phone or tablet.
Better to use 3rd party apps on iOS or Android rather than Apple Books, Kindle App or Kobo App etc unless you don't know how to import to Calibre stripping DRM.
Overdrive and Borrowbox Library apps or Adobe tools needed for Library loans. It's not reasonable to manage them via Calibre, only books you buy or are Public Domain.