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.
Above option is the best if you can order the part.
Also achievable by filing 08-20
I would caution though that parts like these are not designed to function with material removed, and I doubt that they have been case hardened or similar as they would be in a real one. So a real risk of breaking.
But defo buy the part if available as this will retain value if you decide to ship on at some point.