PostgreSQL is really the best open source SQL database in my eyes.
It is so robust, has standard SQL compliant high availability features and tons of other things without leaving SQL standards. It is just great, awesome.
We used AWS serverless stack for some prototype project. Thus, we somehow had to use AWS IAM.
It was no real decission and I don't like it as it is complex, has a bad UI and is hard to understand with a buggy online editor.
Nowadays I would prefer Azure for such a project due to lower cost and ActiveDirectory integration.
extendable
I knew Nagios for decades but it was really outdated (by its architecture) at some point. That's why Icinga started first as a fork, not with Icinga2 it is completely built from scratch but backward-compatible with Nagios plugins. Now it has reached a state with which I am confident.