You may be interested in a multi site configuration.
There’s two big problems you’re running into: Discourse isn’t ready for Kubernetes, so custom code is required. And you’re edging into what the Discourse team does to make money (hosting a large number of forums), so the level of support you’re getting will drop off.
My advice? Do a multisite configuration with static scheduling onto VMs, entirely outside your cluster. (Or a Service Type=ExternalName pointing to the VM to keep the same Ingress.)