We also lack the biological knowledge to build stable closed ecosystems that last for years with minimal degradation of essential elements and processing systems, the engineering knowledge to build complex machines that last for years without any significant failures while being ablated by space dust and occasional rocks, and the political and psychological knowledge to keep a crew sane, focused, and functional.

Apart from that - solved problem.

> to build stable closed ecosystems

Sealed terrariums? Unless you mean something more complex than plants, soil, sunlight, and water?

https://laidbackgardener.blog/2019/10/14/the-worlds-second-o...

"That means it has been growing inside its bottle with no additional air, water or fertilizer for 47 years."

... and once you get to the destination you have to build your whole industrial infrastructure from a tiny seed.

It's bad enough to do on Mars (or an asteroid or other solar system environment) where you can get spare parts sent over in a few years and ask ground control for answers to problems, have them simulate things on Earth or in cislunar space and put a lot of engineers on it. The more I think about it the worse it seems in our solar system -- but interstellar travelers are truly on their own.