I think you have missed one consideration. UBI addresses the demand side, that is, the income people have. But income is meaningless without context- the supply side needs addressing. As it is, no country is fully self sufficient and must trade goods and services with one another. The US happens to be in a more advantageous position in some regards geopolitically & in terms of natural resources, but AGI has to address the issue of supply or the income is meaningless. More income with the same supply simply translates into inflation in the cost of goods and services, requiring ever increasing UBI values which won't ever catch up with cpi, ultimately resulting in runaway inflation. What is needed is a much larger supply of goods and services, keeping prices deflated and resulting in abundance. UBI without this is meaningless. AGI can't simply take over most jobs, it much create an over supply of the things we need, acquire the resources from different geographical locations and distribute them efficiently. Once we have such breadth of precise control over global economies, UBI could be rendered a little meaningless, as the cost of goods and services drives towards 0.

Just something to think about, since people often miss this supply point when discussing stimulus & UBI like measures.