This is a very simple mouse with no/hardcoded software (only registers for basic configuration): https://www.nager-it.de/en/maus

It has an “as fair as possible” supply chain and can also be self-assembled. Detailed documentation is here: https://www.nager-it.de/en/informationen/material

This (the keyboard thing) is is obviously false, so I have to assume the whole story is

Specifically for eduroam, I assume you can be tracked anyways, since you have to authenticate with your personal credentials, right?

1. I usually do the updates in the evening when turning off my laptop. I click “update all” (or so) and then choose “shut down” in the selection that appears at the bottom. Then I can leave the laptop and it will automatically reboot to install the updates and then shut down completely.
2. Yes, that number of updates is normal
3. I don’t know of a way to (un)select specific “system”-packages within Discover
4. The updates are usually fine. I can’t think of any case where a routine update caused trouble for me. I prefer getting the security updates (and other improvements) quickly. Just once, I intentionally delayed the updates for a few days before an important deadline, where I needed my computer to work and wouldn’t have had the time to deal with issues

You can remove the bar for Firefox: Right click on the toolbar -> “Customize Toolbar…” (or Burger menu -> “More Tools” -> “Customize Toolbar…”), then in the bottom left uncheck “Title Bar” (This works in KDE, at least)

I’ve last seen it last month. And I have an old chat, where FluffyChat and (“old”) Element show all messages by now, but Element X can’t decrypt many and both Elements report that they can’t guarantee the authenticity of many messages (even my own). For a long time, my chat partner could only read messages I sent via FluffyChat but not those sent by Element. I have not checked if that is still the case.

For public transit routing, I usually use the app of the local provider. But especially (not only) for Germany, Öffi is an open source alternative for them, that uses their APIs, I think.

FairEmail, email app for Android that has every feature I can imagine. Available on FDroid

OsmAnd, mobile app for navigation and maps. Offline, based on OSM, can do anything. I use it for checking out and bookmarking places, finding POIs and pubic transport stations, routing (especially for bike and foot), measuring distances, and so on. It can also show and record tracks, do car navigation, edit OSM points, and more stuff that I don’t use. Pro/plus/full version free on FDroid