Generational ships would of course need to be very large, but I doubt it would need to be as large as you think. And it doesn't need to look like metal tubes. Many northern cities have extensive underground or between-building pedestrian bridges and large shopping malls, etc that can provide quite a lot of variety and the feeling of open and green spaces that is pretty attractive during long cold winters. Whether that's 'enough' to avoid mental health issues in a permanent setting is of course a different story, but that's just one of thousands of problems that would need to be solved before that ever comes close to reality :)

Yeah, this is the problem though - ironically highlighted by my still-maintained love and hope for Starship: Beyond Earth orbit, the energy requirement to move even small tens of tons of useful life-sustaining mass is incredibly expensive.

Like, to get a useful amount of people to Mars would be... the wealth of a first world nation for tens of years. Even using nuclear engines.

A generational megaship travelling at some small percentage of c to a nearby useful star (not even the nearest ones, which are all a bit shit)?

There's just nothing within our current projected reality that could even begin to accomodate that possibility.

Never mind the fact you'd need redunancy, and at least a few hundred years of testing to ensure that whatever mega project you could ultimately send wouldn't simply get vaporised halfway through, from realities unknown.