Ich habe es nach zweiwöchigen Horrortrip tatsächlich gelöst !
Es war die Fritzbox !? (Das hätte ich nie gedacht!)
Bericht anbei (in Englisch):
OMG I fixed it !
You won't believe what the root cause was: the Fritzbox 7590 !
The answer from Kathrein brought the solution.
They said it's either a defect EXIP or it's within the network.
So today I setup a switch right next to the EXIP and tested TVH (and VLC) from there.
And I didn't get the problems.
So I setup another SD-card for RasPi with an IPerf3 server to test step by step (=wire for wire) where the packet loss came from. Because in the cellar itself I got 0% packet loss EVEN at 800MBit speeds (with 8 parallel streams each being 100 MBit).
And voila I got 0% packet loss just until the Fritzbox (which I was using all 5 ports on, as a star point for my whole network).
So I tested from the other directions aswell (office e.g. with 2 switch after the Fritzbox). And what shall I say? 0% loss just until the Fritzbox.
So what I did is to just buypass the whole Fritzbox with another 8-port switch just ataching it to that switch for Internet and the USB-disk @Fritzbox.
And not only do I get 0 paket loss in all rooms, also my SAT>IP finally works without ANY issues.