Where would someone get all of the raw materials to build such a structure? If it has the surface area of that many worlds….it seems massively unrealistic that something like this could be built.
Depends how thick the hull is. It need be only extremely thin compared to its overall diameter. The assumption is that you'd dismantle an entire solar system to make a Dyson sphere. And yes, that would take quite a long time, but if you're in the position to build one then this is likely not a big issue.
The total mass of all the planets, major moons, asteroids etc in our solar system – everything apart from the sun – is approximately 2.8e27kg. Assuming this mass distributed evenly across a perfectly spherical shell of a uniform density equivalent to that of Earth, the "Relics" Dyson sphere – diameter of 200 million km – would have a shell approximately 10m thick.
You could also do "
star-lifting" to remove material from the parent star. As smaller stars burn through fuel more slowly this would actually increase its lifespan, while decreasing its energy output – but also remember that the total mass of our sun is about 710 times the combined mass of the rest of the solar system, so there's plenty of spare material to be had to bulk out your shell if you think it's too thin.
The bigger issues with a shell-style Dyson sphere are more to do with what happens
after you build it.
Firstly – the net gravitational attraction of the central star and the shell is zero, which means the shell may drift over time – the star will not automatically remain centred. Ultimately without the ability to correct this the star will ultimately collide with the shell, destroying it.
Secondly – without magic
Star Trek gravity technology the only way to generate gravity inside the Dyson shell will be by spinning it. Which means only a narrow equatorial band will be habitable – the rest of the shell will be too "steep" or have too little gravity to be useful. (This is where Niven got his idea of
Ringworld from – it's basically a Dyson shell with the uninhabitable bits north and south of the equator removed.)