If you use a fancy official VPN client from Mullvad, PIA, etc, you won’t need this since most clients already have a kill switch built in (also called Lockdown Mode in Mullvad).

According to the researchers...

The result of this is the user transmits packets that are never encrypted by a VPN, and an attacker can snoop their traffic. We are using the term decloaking to refer to this effect. Importantly, the VPN control channel is maintained so features such as kill switches are never tripped, and users continue to show as connected to a VPN in all the cases we’ve observed.

Killswitches are insufficient protection since the TunnelVision attack never disables the VPN tunnel. The TunnelVision attackers are instructing your physical layer connection to route everything through a node of their choosing rather than killing your VPN connection, and since the VPN connection never drops, a killswitch will never engage. The VPN stays up, thinking it is doing a good job, but in the meantime your network interface has been instructed to route no traffic through the VPN and instead route everything to the location of the attacker's choosing. I have heard that a couple of VPNs think their clients are not vulnerable here, but I haven't seen independent conclusive proof one way or the other yet.

I suspect that your "Solution" also fails to mitigate the issues in TunnelVision because it allows LAN access to the physical interface. In a TunnelVision attack the hostile has to be on your LAN (or rather the same LAN you are on since I suspect that "The coffee shop wi-fi" is the more likely network for an attack like this) already, so if they're going to tell your interface to route traffic somewhere else, in all likelihood that somewhere else will already be in the same LAN you are and their exfiltration will be allowed under your configuration.