System design is not just about the game (individual tools), but about the meta game (flows of data, interconnected abstractions, navigating problem space). There is no rote checklist process that will reliably pick the right tool.
You do need to learn about tools at least superficially, but when you learn how to build the right mental models for your problems, that's when the whole picture starts to become clear and you will just "see" how the right tools will slot into your problem. Then you can deep dive into those tools.
I'd highly recommend starting with Bret Victor's demo, Up And Down The Ladder Of Abstraction: http://worrydream.com/LadderOfAbstraction/ (view on desktop) to start building the "abstraction muscle".
Then it will become more apparent what constraints might lead you to choose a message bus with a RabbitMQ broker instead of making internal HTTPS calls, for example.
[But really, as to your final paragraph, just use Postgres until you can't anymore]
You do need to learn about tools at least superficially, but when you learn how to build the right mental models for your problems, that's when the whole picture starts to become clear and you will just "see" how the right tools will slot into your problem. Then you can deep dive into those tools.
I'd highly recommend starting with Bret Victor's demo, Up And Down The Ladder Of Abstraction: http://worrydream.com/LadderOfAbstraction/ (view on desktop) to start building the "abstraction muscle".
Then it will become more apparent what constraints might lead you to choose a message bus with a RabbitMQ broker instead of making internal HTTPS calls, for example.
[But really, as to your final paragraph, just use Postgres until you can't anymore]