As others have said, ultimately what you are doing is very productive-looking procrastinating. The only way to start developing is to, well, start developing.
You don't need a domain name until your code must make a call to a domain, and you don't have any in your pre-existing dev stable to use. At that point, and not before, register one for scratch work. Don't worry about coolness or appropriateness at launch. That comes later.
Picking a language - pick one that works that you already know. If your goal is to make the code go, it isn't time to learn a new language. If your goal is to learn a new language, then your project is a means to that end, not the goal. If you want to MakeTheThing(tm) use tools you currently know. If you want to LearnTheLanguage(tm) making a thing is a fine way to approach it, but the Thing isn't the output.
Learning "best practices" and designing TDD, and user stories, and all that, while valuable, ARE NOT NEEDED TO MAKE THE THING! Making the thing gets you started. When any of the actions you listed become a blocker to making the thing, then, and only then, address it.
You don't need a domain name until your code must make a call to a domain, and you don't have any in your pre-existing dev stable to use. At that point, and not before, register one for scratch work. Don't worry about coolness or appropriateness at launch. That comes later.
Picking a language - pick one that works that you already know. If your goal is to make the code go, it isn't time to learn a new language. If your goal is to learn a new language, then your project is a means to that end, not the goal. If you want to MakeTheThing(tm) use tools you currently know. If you want to LearnTheLanguage(tm) making a thing is a fine way to approach it, but the Thing isn't the output.
Learning "best practices" and designing TDD, and user stories, and all that, while valuable, ARE NOT NEEDED TO MAKE THE THING! Making the thing gets you started. When any of the actions you listed become a blocker to making the thing, then, and only then, address it.