How to create secure credentials with custom connection options?

I have this code at the moment that I use to increase the frame and window sizes:
```
class CustomChannelCred extends ChannelCredentials {
constructor(callCredentials?: CallCredentials) {
super();
}

compose(callCredentials: CallCredentials): never {
throw new Error("Cannot compose insecure credentials");
}

_getConnectionOptions(): any {
return {
settings: {
// This will increase the http2 frame size. Default is 16384, which is too small.
maxFrameSize: 4194304,
// The initial window size set here is overridden later. We need to patch the grpc-js library to allow this.
initialWindowSize: 4194304,
maxHeaderListSize: 8192,
enablePush: false,
maxConcurrentStreams: 0,
},
};
}

_isSecure(): boolean {
return false;
}

_equals(other: ChannelCredentials): boolean {
return other instanceof CustomChannelCred;
}
}
```

Previously I was connecting to an upstream without SSL but that has changed. I can successfully connect to the upstream if I scrap all of that and just use `credentials.createSsl()`, but then I'm not setting my custom connection options. Making `_isSecure` return true also doesn't work.

I've tried a variety of ways to create a custom channel credentials class, composing with CallCredentials, etc. but none of that seems to work. I'm reaching for some kind of `compose` function that works with another instance of ChannelCredentials, but it doesn't exist.

I looked at these docs and they didn't really help either: https://grpc.io/docs/guides/auth/#nodejs.

In short, how can I modify the connection options on an instance of ChannelCredentials?