I have been using EURKey for a few years now. You might like it. https://eurkey.steffen.bruentjen.eu
Besides, Romanian language is not fully supported, lacking ț and ă, and Polish too, lacking ą and ę.
My problem is a proper layout on a German keyboard, not some weird Umlauts on a US one :P
I think, because I only ever need umlauts, I still prefer "German (US)" which only adds umlauts to the US layout.