Chirpstack uses what is called a “shared mqtt subscription” which means each MQTT event will only be read by one of the Chirpstack instances connected to it. This allows you to very easily spin up multiple Chirpstack instances pointing at the same broker and it will just work. Throw a load-balancer in front of them and you’re done.
I am also working on a production Chirpstack deployment right now so you might be able to learn a trick or two from my plan:
Another consideration with your setup, where gateway bridges are on the server rather than on the gateway, is what are the gateways sending over the air? Do you have a way to secure the UDP forwarder, or are you using basicstation with TLS?