Unfortunately creating your own image is really the only feasible option to ensure it's disabled entirely on the router.
Unless your ISP is providing you with the IPv6 connectivity then this won't be happening. And if you are being provided with IPv6 connectivity that's even more reason to not be disabling it.
Removing/disabling IPv6 on your router won't necessarily stop internal devices using IPv6 to communicate between themselves. You will have to disable it on each device separately.