The driver's side was unsurprisingly worse than it looked at first glance.
Here it is, sawzall-cut back to good metal.
FORTUNATELY! The edges of the cut are where the metal is thicker/structural and so slower to rust away. So there's a good "border" to weld to, almost all the way around.
That section running down the middle of the footwell is an underbody "frame rail" - a box channel stiffening element, also made of thicker metal. It needs a bit of first aid.
Based on what I'm seeing, I bet someone probably sells pre-fab floor patch panels for this car. Of course, I'm just going to cut & paste scraps of sheetmetal I have laying around.