So; it was the WinTAB after all. Just came home and installed a WinTAB driver (the first one I could get hold on. For anyone in the future having the same issues: look for gqgt01ww.exe. You'll find copies all over the Internet)
After the restart of Windows 11 fired up Xara Designer and, lo and behold, finally a working pressure pen again!

Thanks a lot for you all. And kudo's for Handrawn, for he nailed it!