Expose an application whose backends listen on port 800 internally, but externally must be served over IPv4 and IPv6 TCP port 700 with high availability.

A Network Load Balancer (NLB) with backend services is the correct choice. It supports both IPv4 and IPv6 external IP addresses and allows port translation, meaning the external port (700) can differ from the internal port (800). Using backend services with an instance group provides high availability and automatic health checks. A Network Load Balancer with a target pool backend does not support IPv6 external IP addresses or port translation. TCP Proxy Load Balancers (both options) operate at Layer 7, not Layer 4 (TCP), and while they support IPv6 and port translation, they are not the most direct solution for a simple TCP port exposure requirement. Additionally, using a single instance (as in one of the TCP Proxy options) would not provide high availability.