Almost like as if people are looking to hire a one-man army to handle the entire department.
Well yea, that's usually the point of full stack. If you want to do something like that, you probably want to work a smaller scale company... Like if you're in a team of 5 people, the situation arises of "Oh sysop thing needs to be done, who to ask? I guess @RonSijm (backend dev) is close enough..."
So to have a junior full stack is pretty counterintuitive. Otherwise the situation arises of "Oh xyz needs to be done, who to ask? - Well be have a dedicated senior backend engineer, a dedicated senior front-end engineer, dedicated senior sysops.... 'Oh let me ask @[email protected], this junior full-stack'" - yea no.
Why are you aiming to be an intern/early-career full-stack engineer? The only kinda company I can think of where something like that would be something with barely any IT, where you're just the "jack of all trades" goto guy for IT stuff - so that you can be the one-man army that does everything
So honestly I'd focus on one area first - backend, frontend, dev/sys-ops - especially as you're mentioning
I’ve wasted most of my time worrying about the stack
Yea that gets even worse when you have worry about the entire stack, and work with an entire stack of components you're not really familiar with. If you're at least somewhat senior in one part - lets say backend - at least you're in a position of "Ok, I have a backend that I'm comfortable about" - "Now lets see if I can make a frontend for it" - or - "Lets see if I can manage to dockerize this, and host it somewhere."
And if you know the fundamentals of one stack-part first (data-structures, design patterns, best practices) - you can apply that knowledge to other areas and expand from there