Today I accidentally stumbled upon a hidden gem in Windows 10, a registry setting that activates high transparency for the Taskbar. It’s the UseOLEDTaskbarTransparency registry value that does the magic.
Update: This registry edit works in Windows 10 version 1903 and earlier. It doesn’t seem to work if you have v1909, 2004, or higher installed. In Windows 10 1909, 2004 and higher, you may try TranslucentTB or a similar third-party utility.