The materials have been shown and proven in theory. It is simply a matter of building the space based manufacturing and infrastructure required. This is like Romans talking about what it will take to build nuclear power plants if they could somehow imagine them. I laid out the economy scale that I am talking about at the outset. This is an order of magnitude, or more, larger total human economic output. An era when this construction scale is not very novel.
Assuming we are talking about an era when Sol has a thriving space industry and the Solar system is broadly colonized.
If we are colonizing the rest of the Solar system, we figured out large scale and pressure vessels already. Once we are building in space with materials sourced from space, most of the problems go away.
Worst case, a ship can use nuclear detonations to both accelerate and decelerate easily within the limits of known materials. This has been thoroughly researched in a US program that was only canned as part of anti nuclear proliferation act. This system can easily handle both ends and traveling faster than any current method. It is a worst case. If we can master fusion, there are other ways as well.
I said generation ships too. I don’t care if it is slow and I think humans could cope just fine on a large enough ship, assuming we don’t find ways to put humans on ice.
I highly recommend checking out Isaac Arthur’s content on YT as he goes though all of this kind of stuff in detail but even further into possibility and future tech. I’m getting much more specific into a time and constraints than what IA does in general.