"How do you recruit people for a journey they will never see the destination of"

This is such a painfully, excuse the loaded terminology, excruciatingly modernist take. You'd easily find more than enough people to crew at least one that fit both all your other requirements AND aren't afraid to think beyond their own lifetimes.

"Society prospers when old men plant trees whose shade they'll never sit in" and all that.

Maintenance

As cool as Comstar and the AdMech are, realistically you'll want to avoid that as much as humanly possible. Instead I think what you'll want is create something that looks like a perfectly even blend of academia, the military and apprenticeship with EXTREMELY integrated curricula that bundle related fields and skills to encourage poly-competence. For example we'd likely have a significantly higher amount of physics in our electrician track and conversely would demand that people who'd normally only deal with code understand the mechanical principles behind the machinery they're programming. Additionally, you'll likely train Nexialists as a way to ensure different departments can communicate with each other even better.

We say it takes a village to raise a child. In space the village is a pressurized can flying through a hail of particles and we can extend that to "it takes a village to raise an expert". Ideally asking "what does [thing] do" has at least 3 different people who can explain it from 3 different angles in a complimentary manner within earshot at all times, because when your civilization is all on its own monomania is a death sentence.

Instead we'd want to start you out with soldering at ten and then keep branching out until you know how to create every piece of a space suit at fifty-five.

Stasis

In absence of methods that effectively create immortality I keep coming back to the idea of "sleeper" ships effectively being tombs in which your organs are stored in canopic jars and your blood is circulated through your body by a machine. That way you can isolate & repair damage much more flexibly and on your destination the body is reassembled better than new. Doesn't let you go indefinitely but quite a bit longer than if we kept you all in one piece.