It isn’t overwritten with every update, but with larger updates it can be. DSM 7.2 is coming up, which is a larger feature update (that also change the docker implementation to a new “container manager” interface. So reason to be careful with this update to begin with. But chances are this one will impact that file as well. I’d say just keep an eye on it after any big update. Or switch to a startup script that you can manage in the DSM interface itself to set these settings. That way you know it will never be overwritten.
I just use task scheduler to run these on boot:
sysctl -w net.core.rmem_max=2500000
sysctl -w net.ipv4.tcp_fastopen=3