Hot take, but if you're earnestly alone your whole life but also understand what the tattoos are and where they come from, they might serve as a form of community, of belonging.

You join those that came before, and will pass on that belonging to those that come next. In that, you are not alone, and maybe that is something worth continuing in your eternal night.

Fiona Apple said it well in her song Relay

Evil is a relay sport when the one who's burned turns to pass the torch

Deep

Great soundtrack

Brightly.

Ion thruster + fusion? Or some futuristic engine.

I think it works by collecting hydrogen atoms over a 30 year period, and every 30 years the collected mass is used for course and velocity changes - it coincides with the clone change.

Maybe it's a nuclear salt water engine, those things have crazy high efficiency if it turns out it's possible to make them.

690 years, clone 23 is about to tattoo clone 24, and they have 30 year cycles.

They do, but clones are cheaper, and have no rights

the body is aged, so let's say the new clone's body is 18-30. That's like 48-60. The technology in the strip might also be imperfect.

Because the implication is that there have been 23 clones with a 30 year life cycle meaning the ship has been traveling for... 690 years.

Stress from Solitary?

Because in general, sci-fi postulates levels of technology beyond anything remotely possible, yet never touches on life extension because it's unnatural.