I agree with nearly all of your points. The stopping problem is the same as the accelerating problem. Assuming near infinite energy reserves, but limited power generation, then the ship would accelerate for half the trip, turn around, and then decelerate for the 2nd half. Depending on the amount of power that can be generated, earth gravity may be possible during the trip (except for the turn around in the middle).

There's no reason that we would expand out at the speed of light in one direction. It's well within the realm of possibility that we can intercept rogue planets or large asteroids to use as long time habitats. Also we can expand in millions of directions at once at sub-light speed. The journey make take a million years, but we'll reach a million places at once.

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.

I'd like to see a pressure vessel made of fibreglass that size... Not happening. Wall thickness in pressure vessels scales

Simple calculator, assuming steel... a 24 km diameter pressure vessel at 15psi is over 13 metres thick steel wall to contain the pressure. https://checalc.com/calc/vesselThick.html

Just the volume of steel required would be astronomical. You might be able to do this out of a similar mass of fibreglass... But forget launching it from Earth (would have to be made in situ).

And, largely, forget the fantasy renderings of what O'Neill cylinders look like -- they are anything but lightweight.

I think it would likely be similar to engineering materials today. The highest performing materials are very difficult to manufacture and we are talking about pushing them near their limits. I think a lot of the substructure will be made in-situ, but I think the most stressed parts of the main structure will require the largest scales of manufacturing. Like Sam Zeloof made a chip fab in a garage, so why doesn't he start producing the next Nvidia GPU. It is that kind of difference here. A lot can be done there, but nothing like what can be done on the cutting edge of what humans are capable of making at out largest and most advanced facilities.