Thanks again for your quick and extensive answer @R1D2. For your first solution I would need a VLAN aware router, which I don't currently have. As I cannot implement it I sadly have to discard it....

@R1D2 Wow! Really detailed answer @R1D2, thank you very much. I've been having a look to the higher end TP-Link switch series, but all of them seem to lack a filter to accept only untagged traffic on...

I have a T1500G-8T V2 switch, using firmware 2.0.5 Build 20200109. I define 3 VLANs as follows: VLAN 1 = System = All ports untagged VLAN 2 = Only LAN = Ports 2-6 untagged VLAN 3 = Only Internet =...