My understanding is that ISS is not self-sustaining even in principle. It consistently needs to be resupplied with water and breathable air as the station continuously leaks it. These resupplies happen about once every month or two. This article goes into quite a few details about what would be needed for actual self-sustainable human space exploration and it looks like there's quite a few engineering challenges to work out.

https://www.frontiersin.org/journals/space-technologies/arti...

Not only that but they have to routinely boost its orbital velocity as there is still a little atmospheric drag at the height.