What is the point of setting an upper limit in Auto-ISO?

No problem. That is why I said it comes down to personal preference.

A typical situation where I would more likely use Av + Auto ISO + minimum shutter speed is when I am pointing and shooting in changing light.

For example chasing the grandkids in a park, especially the 3 year old, who will be in the shade under a tree one second and out in the sun lit open area the next. I would set the minimum shutter speed to freeze motion when she is in the shade. With a minimum shutter speed set, the shutter speed would automatically quicken to suit the extra light in the sun lit area. I would have to constantly change shutter speed if in manual mode.

I tend to use manual + Auto ISO when the lighting is fairly constant and/or when using a tripod.

Set the minimum shutter speed to a function button.

Now how it would be different from M?

I can't think of any reason why we couldn't be shown that all of the time that the EVF or LCD is active. There are always settings in place before we half-press.

... assuming that you did not normalize brightness. If you normalized it, the 6400 should be less noisy-looking with many cameras, and I think that normalization is a "given" in this context.

Many museums use very-warm broad-spectrum lights that create a lot of near-infrared which can put a magenta/purple cast on the objects based on their NIR reflectance. It's not an easy thing to fix. People who have cameras that are more sensitive to NIR need to be aware of the problem with "very warm" broad-spectrum lighting. This may be addressable with filters, but filters will lose some visible light, too, and the light may already be too low in such places.

I have found that the highlights in many dynamic scenes don't change much. In the two examples you posted, it doesn't look like they will.

I think the only advantage of A mode is saving time in post.

Lr definitely does a better job of pushing than pure multiplication.

Your arguments are based on the assumption that the minimum shutter speed is set to the absolute minimum that is tolerable, right on the edge of risking motion blur.

I'd rather increase that minimum shutter speed and set the max ISO a bit lower, hence having a better chance of avoiding motion blur at lower ISO, yet not push the camera to max ISO by doing so.

On my Nikon Z9 you can only change ISO value, not the minimum shutter speed. On Z 7II and Z6 II you cannot get to the ISO sensitivity feature at all via Fn button

In M mode, the shutter speed does not change and can lead to overexposure once the base ISO is reached.
In A mode, the camera's metering will increase the shutter speed when necessary to prevent overexposure.

I use a completely different exposure philosophy if it is possible to use base ISO.

On the other hand if your minimum shutter speed is high, it might lead to unnecessary high ISO.

It is unfortunate that we cannot assign Auto ISO (or d9 settings) to function buttons with Nikons while other manufacturers allow it. I have added it to MyMenu,

Less DOF than what other option? Less than with a slower lens, AOTBE, but not less DOF than a FF camera and lens giving the same photon noise and image-level diffraction. It's always down to the pupil size, and distance from the subject, for these three parameters for a given subject in given light and a given exposure time.