I feel like the statement “certain to get me hired” and “reasonable for a junior to accomplish” are contradicting.
For example, you can certainly get hired at Facebook, if you build 90% of Facebook by yourself and start taking away market share. If you replace Facebook with any other software company and just start “beating them at their own game” and walk in and ask for an interview, your basically going to get “bought out” for the price of a single developer (yourself).
I’d say this is probably the only certain way, but is totally not realistic haha. There isn’t really a certain way to get hired at any company (what if they aren’t hiring at all?)
I agree with @camelcamper, having a wide range of applications, or specific applications for where you are applying is probably the best. For example, if you are applying to Facebook, and you write an app that looks feels and works similar to Facebook, or part of Facebook, like Instagram, or just Messenger, you will look good. If you use the same stack/tools as Facebook you will look even better.
So don’t use Angular or Vue for your front-end if your applying to Facebook, and if you can find out the backend stack Facebook uses, use the same stuff in your project.
But since you should apply to more than just Facebook, being a jack of all trades is better than a specialist when job searching. You will appeal to more companies, be ready to learn more (in depth) and gain more working knowledge of more things. You should also do some research into what jobs are near you, and where you are applying. If most jobs are asking for Angular, you won’t get much use if you just learn React. Now, if you learn both your better off, but I’d consider it redundant skills, since you will only use one of them in your job most likely.
Goodluck and keep building