Came here to comment this, there's a ton of awesome stuff around Postgres that using RDS makes impossible. Kinda stinks, but I don't have the time/expertise to manage a Postgres Instance on EC2.
If the extension availability is the main concern, I'd recommend the open source StackGres [1] operator, which has, possibly, the largest Postgres extension catalog [2] available.
[1] https://stackgres.io [2] https://stackgres.io/extensions/
Disclaimer: founder of OnGres, the company behind StackGres