Looking at this, it doesn't seem like balancing between links is what you would want to do. You would be better off building two tunnels and balancing between them. This would give you future flexibility for an SD-WAN type control over two different circuits. If you are doing a load balance, you would have to re-configure this later on and ultimately re-architect this if you moved say Eth4 to a different provider to mitigate unreliable consistency.
To answer the question, you could use a load balancer that keep track of the health of the endpoints its balancing to and will always balance to from one device to the VPN server to the same node (provided that it is healthy). Alternatively, you could have the box check into a VPN registry that keeps track of healthy VPN endpoint and return the server to connect to. Both of these options are not great and its easier and more flexible to create two tunnels and balance between them(as shown above).