Another approach to consider, that's so straightforward and quick that most will consider it boring and overlook it.

- kill services with low usage

- downgrade instance size

- downgrade instance type

- merge databases

- schedule services with obvious usage patterns to shut down when not used

- use EC2 spot instances

Most importantly, it requires an aggressive culling mindset. If drastically reducing the AWS bill means staying afloat, then make bold choices.