Hi bud. You just need a semi ‘disconnector’ to achieve what your after. In the diagram it’s part 08-11, but that’s the wrong type as it allows full auto. German spec HK417s have semi auto disconnectors. Umarex ref is 80.20.0022, probably order direct from VFC.
Unfortunately I don’t have a 417 no more otherwise could have sent you some internal pictures.
I’ll look in my 416 as I assume there not too dissimilar.