What is the opinion of Reddit about the
German Keyboard Stickers with White Lettering ON Transparent Background?

>With Duolingo I can just tap on the umlaut vowels and double S, but it’s not a great solution and won’t be present if I’m typing on other websites/applications

A physical keyboard doesn’t make any difference, you need to install and switch to the appropriate layout in your settings.

You could use the US international layout and type Umlauts like this or install the German one and buy some keyboard stickers.