I found an audio interface from Behringer - the UCA222 (for about 23 EUR - as from Jul/2020) which looked pretty promising and cheap. It has input and output RCA sockets, as well as a 3.5 mm headphone jack with a volume control wheel and of course S/PDIF via TOSLINK.
Fortunately I found a block diagram of the audio chip (CM6533) used within the UCA222. That showed me that the digital output is almost a "pass-through".
I spoke with the Behringer team. They told me most of their devices are class compliant - so Linux should work too.
That's what I finally bought and it works quite smoothly. The AD/DA components are probably not the best (haven't tested them so far) but that's why I connect it via a S/PDIF switch to a high quality DAC.