Hi Lawrence.
Thanks for providing this information and asking in our forum. I am trying to understand what your current goal is, do you want to proof that you can run a certain amount of process instances per timeframe? Because what you actually do is stressing the platform as if you need to get high throughput. And depending on how you configure it exactly, you might run into Optimistic lock problems, ddeadlocks or the like. This is generally tunable, and I am confident that you can reach a stable level (see also Camunda Best Practices - Performance Tuning Camunda) - BUT: Do you really need this?
If you need to run 1Mio instances/year, that is less than one instance/minute, which is actually not a high load for us. We run up to 1000 instances/s successfully in some scenarios (with tuning as discussed). So it might be, that you mix up two things: “Can Camunda run 1Mio instances” with “Can Camunda run 1Mio instances in a very short time frame”?
Another thought: If you really need high-throughput it might be worth looking at Camunda Cloud, as this is optimized for these use cases, see e.g. GitHub - camunda-cloud/camunda-cloud-get-started: Camunda Cloud Get Started Projects. WDYT?