It would require a lot of strong social structres. Because a lot of important tasks (like maintaining the floor cleaning equipment in the robot-tractor tyre factory) just really isnt that interesting but still needs to be done. I can see ways motivation can come from others. I'd argue my motivation to persist in my work isn't (directly) because of the money I am paid but rather due to social effects of my colleagues, and the money only contributed to building up this colleague social structure in the first place (alongside the potential value our work brings being an almost equally powerful motivator)

keeping these things going when people lose interest will be a challenge.

I would say your best case requires technological advances that won't necessarily happen. In a truly postscarcity future I don't think we'd have any use for UBI. I do think UBI would help empower everyone to work on hard problems in a way that maximizes our odds of getting there, but I'm envisioning it existing alongside scarcity. We're just thinking about very different scales of time.