How to schedule multiple tasks on AWS ECS using different env vars?

Let's say I have task definition on AWS ECS and want to schedule it to run as multiple instances with different env variables (~20 parallel tasks). I have some ideas how to do that, but not sure which one is correct.

1. Create multiple task definitions with different env variables, which sounds not really effective and silly
2. Create multiple targets for scheduled tasks, but its number is limited to 5 ones, which doesn't work for me.
3. Create 20 container overrides, but I didn't found a way how to do it using the user interface, and not sure it's correct too.

Could you please give me an idea? Thanks