One note on reserved instances: they can be sold for EC2. So I actually work for a company that buys and sells RIs on behalf of customers and we buy 3 year no upfront instances and then sell them as soon as a month later. You have to have some idea of the liquidity of different instance types to do this well however. But c5, m5, r5 (common instance types) and us-east-1, eu-west-1, us-west-1 (large regions) are pretty safe.
This is actually a surprisingly good article. Probably the most important part of taming aws costs is cattle not pets.
Yeah, the best thing that would improve this article would be to add a second one going into more aspects. :)
I would love to see advice about minimising egress costs, for example.