I realised that I should probably explain why Germany is so particularly slow:
It's not engineering challenges; unlike Austria, Switzerland, or Denmark, Germany doesn't have to build long Alpine or undersea tunnels. From an engineering perspective, the German sections should theoretically be rather straightforward upgrades.
The real problem is that these sections are densely populated and due to Germany's federal system, every affected actor gets a say. Every state, every municipality, every environmental NGO, and every farmer can open up a litigation process. Germany also splits every corridor into many smaller approval sections, initially as a meant to speed things up, as its easier to get approval for 10 km than a 100 km segment and you can start building on already approved sections. However, this also results in 20 different sections all in a separate legal hell, and every time a section gets redesigned you start the process again (new geological surveys, environmental impact studies, court cases, etc.).
There's also a big cultural issue. The Germans have become very litigious and risk averse, becoming very reliant on consensus. Whereas the Swiss and Austrians are proactive with rail infrastructure, the German state (and DB) has also fostered an "only fix it once it's a problem" attitude. This is obviously less than ideal when projects take decades to realise.
Despite, pressure from their neighbours, there's still a lack of political will to push things through. Nobody wants to rock the boat too hard, and there are also powerful lobbies to consider. To most politicians it's not worth risking their careers over, and it's not like they take the trains anyway.
Finally, one must remember that most of these projects were conceived in the 90s or earlier. Including all the additional delays, it now takes Germany more than half a century to complete relatively simple infrastructure upgrades! The worst part is that none of the example projects even have full approval on all sections yet!