I'm going to assume since you're processing off a queue, the lambdas are not serving a waiting user. If you do the work to orchestrate onto Fargate, you might as well go the rest of the way and move to ECS. Then you can use spots, where real savings kick in.

Someone mentioned step functions above. All of our steps run on spots. We also have some tasks like you have that read off queues and do processing, which also all run on spots.