Regarding your first paragraph: Agreed.
Regarding your second paragraph: You're looking at this as a measurable global issue that will happen at a single time resolvable by a single government entity, but that's not how it will unfold.
It won't happen overnight, so there will be no point for those options to be evaluated. Let me pitch you (what I see as) a more realistic option:
Some countries will try to implement UBI. They will encounter the same problems opponents have been warning about. The productive will (over time) move to countries that don't take so much of their wealth. The people receiving more than they're contributing will have more children, will have more votes, and will vote to take more and more from the decreasing minority. Even if you can make food truly post-scarcity, housing, healthcare, and other goods/services are not.
If everything is truly post-scarcity: It'll be no problem. Those countries won't really need the wealthy. But I'm not sure true post-scarcity actually exists, and I think (especially in the US) UBI would be a moving target that only ever goes in one direction.
You're right, though, that I don't really have a solution if (e.g.) 80% of people no longer have a meaningful way to contribute to society. That may be fundamentally impossible to solve. But that may never happen and we may never have true post-scarcity, so we shouldn't be redesigning society based on the premise that wealthy people are an endless money tree, and if it's wrong, society collapses.