I was driving yesterday, and the sun appeared in the intersection between the windshield and driver-side window. When I pulled down the shade, it blocked only one side and the sun shone through the other window, even when I twisted the shade to the window. How can I block the sun from getting in my eyes if it appears in both windows?
In lighting (stage, film, photography, video), a lighting technician often wants to avoid light falling on something specific. A flag is used to block light. Another term to avoid light spill is a gobo (from something used as a go-between). There are others. Your visor is an example of one that almost works most-of-the time.
I propose you custom-make a gobo for extending the sweet-spot of your visor.
Make a card flag to block the gap between what you have and what you want.
I picture a piece of light card-stock, folded/doubled in half, and hung over the top of your visor.
Hold it in place with a binding clip or a clothes pin.
The problem with the sun and driving is that everything is moving. Unless you are driving on a highway in the midwest (level & straight), there will be a corner, an elevation change, or something that makes that one spot be in a different place, in a minute or so.
Normally I put my elbow on the windowsill and block the sun with my left hand. (left hand drive) The windowsill is normally perfectly positioned to support my arm comfortably while blocking the sun with my hand.
This is really easy to move as needed. I always have it with me regardless of what vehicle I am in.
In most (almost all?) cars, the A pillar is wide enough to make sure the sun doesn't shine simultaneously in two windows. But if the road has some small curves, you might face the sun alternating between the two windows, that's true.
Could you try to sit somewhat more upright or reclined than you're used to? That might be bad for your back but it'll only be for a short period. A greater problem might be the mirrors which aren't in a 100% ideal position anymore, but you should have some leeway there.