Should I use a separate demo environment or feature flags for custom features in B2B SaaS?

I'm a backend engineer at a B2B startup, and we're in a scenario where sales sell features to specific clients before they're fully released. Currently, I'm working on a release that involves three features, one of which—a survey feature—has already been sold to a customer. Our business is considering deploying a separate demo or stand environment to showcase this survey feature to the customer. I'm trying to figure out the best approach: should we deploy just the survey feature to production ahead of schedule, hide it behind a feature flag, or enable it only for that specific customer? The concern is that using a feature flag adds complexity to our codebase and might complicate hotfix deployments. On the other hand, a separate environment could be safer for production. I'm eager to hear your thoughts on what's typically preferred in B2B scenarios, whether my assumptions about these two approaches are accurate, and what questions I should ask the product owner to help reach a decision.