As a senior dev: it will take years of experience and constant learning to get to this point, but it is attainable. You don't have to be a master of each part of the stack and each tech within it - you just need to be able to do enough, and solve the problems you face. You can build a professional SaaS product with a simpler stack as well.
You take a project like this one step at a time. Some bits of it are relatively easy - setting up a few postgres servers doesn't take much knowledge. ElasticSearch is a little more obscure, but for the most part, things like this are running a few commands, and setting up a few config files with the help of docs and google. Same for Redis, nginx... etc. Which isn't to diminish devops - you can dive deep into each of these configurations and develop pretty complex setups, but by the time you actually need to you hope to be making enough money to pay someone else to do it.
You won't get everything perfect all the time. You'll have to revisit parts of the stack and tweak them. But you can take it a day at a time and do what you need to do.
You take a project like this one step at a time. Some bits of it are relatively easy - setting up a few postgres servers doesn't take much knowledge. ElasticSearch is a little more obscure, but for the most part, things like this are running a few commands, and setting up a few config files with the help of docs and google. Same for Redis, nginx... etc. Which isn't to diminish devops - you can dive deep into each of these configurations and develop pretty complex setups, but by the time you actually need to you hope to be making enough money to pay someone else to do it.
You won't get everything perfect all the time. You'll have to revisit parts of the stack and tweak them. But you can take it a day at a time and do what you need to do.