Have you found the effort of setting up and maintaining existing solutions, such as AWS Instance Scheduler or Cloud Custodian, worth the savings you generate with them?
Have you found or built yourself solutions that your development staff is happy with? Can they for example easily adjust the default schedules when needed? Can the solutions be further integrated with e.g your CI/CD pipelines, so that environments can be brought up/down on demand?
Full disclosure: I am thinking about building something for this space.
There are some AWS tools to do it, but they cost too, and of course cannot spot many ways to save money, but there are.
On of my customer told me: we can easily pay you a full month of the saving we get after your cost optimization work (it ended up a lot of $$ :), but the hard part is that to be able to help them save money, you kind of need full root access, and you wont be able to easily get it unless the company is small (or not structured).
Use reserved instances rather than spot pricing.
Multiple tools/services in the space you describe.