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

Thank you, you precisely describe scenario of using auto iso.

It is easier to fix excessive noise than motion blur (DxO, Adobe Denoise).

I think whether you use M + Auto ISO or Av + Auto ISO + min shutter speed simply comes down to personal preference. I use both modes depending on the situation.

What I don't understand is why you would set -1EC if using M + Auto ISO with a Max ISO set.

On my 90D -1EC with M + Auto just lowers the ISO setting the camera want to set by 1 stop. The 90D has always been 'smart' enough to not set ISO too high resulting in highlights being clipped without the need for a minus EC.

Well, I've found that it's not a pure black & white, or a work or fail type scenario. Even confining yourself to one single format -m4/3 in my case. I'm not that keen in witing up a huge report into what's the "best", because it varies by camera, raw converter, scene, & personal preference.
What I will say, is that from the GX8 onwards, the in camera raw conversion has gotten better with each model. Once again, that's not a hard & fast "rule" as my progression has been from GX8, to G9 & then down" to the G100. Which, surprisingly, I like way more than I thought I would. Despite being one of the most despised cameras on the interwebz.
As Danno pointed out, what I've found that seems to please me, is to use the maximum aperture that will give acceptable DOF for the scene, minimum ISO that still provides the desired or required shutter speed, and try to avoid large exposure/shadow pushes. I'm also shooting raw/jpeg, I've found that the in camera jpeg conversions are pretty damn good from the G9 & G100, often I'm not able to better them with raw converter software on my PC. Once again, depending on the type of scene, whether it's a shallow DOF effect or trying to get everything sharp from foreground to background. It does make a difference. I could care less about a bit of noise in a shallow DOF shot with an out of focus background, so a different technique or settings will be used as opposed to a landscape shot. In saying that, I'm using SilkyPix Developer studio Pro 11, and there's probably more advanced & capable software packages out there.
I've played with DXO pure raw (standalone) a bit, doing the underexpose to protect highlights thing, then pushing exposure/lifting shadows later, but I've found you can get some weird contrast effects as well as colour shifts happening at times. Not every time, but it does happen. Happy to just let some background highlights blow out, and concentrate more on the subject. Not sure if that's an issue with the program itself, or that the developers simply haven't invested the time in the m4/3 side of it, simply due to the format being such a small part of the overall market.
Like I wrote, these are not hard & fast rules as such, no doubt they'll change as both camera's and raw developers continue to evolve, I'll just keep tinkering & experimenting to see what pleases me the most :)

With M + Auto ISO, I can change the shutter speed quicker than with A + Auto ISO (need to go into menus). However, when the light is bright, and I may hit base ISO, A + Auto ISO is safer. With M + Auto ISO, I can accidentally clip the highlights as the shutter speed does not increase as with A + Auto ISO.

To increase highlight headroom.
With Auto ISO and at higher ISOs, -1 EC will only reduce the ISO, which has no negative effect, especially if working above the DCG point.
Every camera I have tried has metering that will clip highlights at some point unless the user intervenes.

That is what I used to do as well, experiment/fiddle with ISO while in aperture priority until it gave me the shutter speed I wanted.

If you have a few spare minutes some time, you might consider doing this quick and simple experiment to see if you can be a little more efficient in getting the shutter speed you want.

1. Put the camera on a tripod and aim it at a scene with constant scene lighting.

2. Set Aperture priority mode and aperture to suit the scene. Let's assume you set f/8. Set metering mode to evaluative/matrix.

3. Now lets assume you need 1/800s shutter speed to freeze the motion in the scene. You fiddle with ISO settings 1 or 2 times and find you need to set ISO 400 for the camera to set 1/800s shutter speed.

4. Now set the camera in Manual + Auto ISO mode.

5. Lock in f/8 and 1/800s and meter the scene.

6. The Auto ISO should set ISO 400, the same value you set manually after experimenting with it in step 3.

So, when I know the aperture and shutter speeds I want I find that either Manual + Auto ISO or Aperture priority + Auto ISO + minimum shutter speed are the most efficient modes for me. I don't need to fiddle ISO to get the desired shutter speed.

Something that was I think not mentioned so far. Many cameras at some point in the ISO range turn to multiplication [increasing values by a certain factor] and/or tagging [instructing the rendering app to increase lightness]. Could the processor in a PC do this better, meaning having the Auto ISO maxing at the change over point could be useful?

I'll give that a try for sure. Most likely the Aperture mode with minimum shutter and auto ISO first. I kind of forgot about minimum shutter there. Good way to find a consistent shutter speed that works to basically freeze what you want frozen, at the required aperture without having to think about the other variables.
I'm about to head off to an evening-night car meet, and because I pretty much shoot with fast primes, setting a minimum shutter for the fixed focal length should work a treat.

Well, I'm out shooting with that setup right now, and it works a treat. Might post a few of the results a bit later. There's MotoGP from Spain to watch, and some beer to be drunk when I get home😁😁

Happy to see it works for you 🙂

It's another setup option you can use depending on the situation.

What if you are hiking and see a bird flying for which you need a shutter speed of 1/2000. Next you see a flower for which you might need 1/50. But in A + Auto ISO minimum SS set to one value. I've found that the only mode which gives you this flexibility is M + auto ISO.

That is exactly why I posted earlier:

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?