One last update from me, i hope it will help future Flashforge-Finder-Users with similar Problems.

Even after some longer Research i didnt find any free Software which supports the Flashforge Finder.
As SimplyFy3d allows you to test the function of ist SW on your 3dprinter for 2 weeks with a complete refund if it doesnt work, i did try that one.
To make it short, it worked. SimplyFy3D does not Support WiFi to the Finder, but USB is good enough for me.
All those models, where i had the above described Problems using the Flashprint SW, now print perfect with the new SW.
So yes, it was a SW Problem within Flashprint when trying to print some complex structures with lots of Support.

The Investment of 150 Dollar solved the Problem. Not nice, but i see it as an invest in the future as SimplyFy3D seems to be a big Player in 3dPrinter-SW and will probably also work with future Printers.