The approach isn't invalid, but seeing as you already have the framework set up to deny all and log for IPv4, the same could be done with IPv6.
That is to say, your router advertises an IPv6 gateway to the global internet, but you then reject it because your VPN doesn't support v6 (sadly). I specifically say reject, rather than drop, because you want that ICMP Unreachable (administratively prohibited) message to get returned to any app trying to use v6. That way, Happy Eyeballs will gracefully and quickly fall back to v6. Unless your containers have some exceptionally weird routing rules, v6 connections will only be attempted once, and will always use the route advertised. So if your router denies this attempt, your containers won't try again in a way that could leak. v6 leaks are more likely when there isn't even a route advertised.
This makes your apps able to use v6, for that day when your VPN supports it, and so it's just a question of when the network itself can be upgraded. IMO, apps should always try for v6 first and the network (if it can't support it) will affirmatively reply that it can't, and then apps will gracefully fall back.
This also benefits you by logging all attempted v6 traffic, to know how much of your stuff is actually v6-capable. And more data is always nice to have.



Fair, though I personally don't let my ISP indirectly dictate what I do with my LAN. If I didn't already have a v6-enabled WAN, I would still manage my LAN using IPv6 private range addresses. There are too many benefits to me, like having VMs and containers be first-class citizens on my LAN, rather than sitting behind yet another layer of NAT. That lets me avoid port forwarding at the border of my home Kubernetes cluster (or formerly, my Docker Swarm), and it means my DNS names correctly resolve to a valid IP address that's usable anywhere on my network (because no NAT when inside the LAN).
I will admit that NAT64 is kinda a drag to access v4-only resources like GitHub, but that's only necessary because they've not lit up support for v6 (despite other parts of their site supporting v6).
This is my idea of being future-ready: when the future comes, I'm already there.