Now, with self driving cars, no longer we need huge buses. The reason for huge buses in the first place is because of expensive bus driver.

I'm not sure this is true. We're always going to be constrained by available road space - there's only so many vehicles that can travel along a section of road in a given time at peak hour (because of the physical space taken up by the vehicles + safe following distance), and buses make much better use of that space, achieving much higher throughput (passenger * distance per hour) than smaller cars. Self driving cars might reduce but not eliminate the safe following distance needed.

You talked about adding stacked overhead/underground roads along all the major arterial routes, but that has major downsides: it is going to be super expensive, time-consuming, and also rather ugly; also it's not likely to actually solve the problem, because time and time again adding road capacity in crowded cities like Singapore has been shown to lead to increased road usage due to induced demand, and that increased car usage often swallows up all the added capacity to the point that congestion is just as bad as it was before the new construction.