You could just setup a separate namepace with IPv6 disabled and move the process into it. I'd be more detailed, but this comment is one eyed and very sleepy, let us know how you go.
Off the top of my head, the only way I can think of is to install steam using docker, and install SF6 on dockerized steam.
Then you've converted the problem to either configuring docker to use ipv4 internally or setting up the container to ignore ipv6, both of which are doable.
There's a good chance it'll be perfectly playable but without trying it out or doing more research than I've invested here, it's not a guarantee.