2. Settings Generator is not for your case, because you have a huge area (almost 1 million km2), and you're walking on a knife edge trying to bypass Unity's memory limit.
Settings Generator is made for common areas to help users choose the right settings and solve the problem when users set maximum settings for small areas, which is much more than the source data resolution, and gives no benefit, only disadvantages.

3. Unity stores position values in float (32 bits), and the accuracy drops dramatically when you move too far away from the zero point.
There is no solution here, only workarounds.
The classic workaround is:
When the observer (camera) on one of the axes exceeds some limit (e.g. 1000), move the whole scene by this limit to the zero point.
You can find more details by googling Floating Origin.

1. Didn't you say that elevation and texture should be downloaded separately last time?
So, I downloaded the elevation with RWT first, and then I used what you call Automator. I thought it was to help me automatically download the textures and link it to the downloaded elevation. But there is no change after its implementation. So can't it download textures automatically? What is it for?
As you said, the method you provide is only to improve the resolution of texture? If that is the case, I will completely misunderstand.
So the real problem is how to download elevation and texture separately. How texture is automatically attached to the corresponding elevation.

2. Yes, this situation is difficult to describe. How to say it? It was when I planned to download the elevation separately that I found this problem. Please take a look at the pictures in my attachment and carefully look at the places I marked. If you do not check the above texture, the downloaded elevation will be rough and the number will be small, which is not suitable and useful.
I believe you can understand it. If not, you can try to download the elevation separately.

3.In addition, I would like to ask another question. I downloaded a large piece of terrain, but after coordinate conversion, the coordinates will be very large.  Drived in the scene by using this large coordinate. The model will shake and blur. I believe you should understand what I said. Do you know how to avoid this problem?

I sincerely hope you can give me some directions. Thanks!

2. Unfortunately, I didn't understand this problem.
If I open Settings Generator and click Apply, Generate Texture is applied in the main window.
Or do you mean that the Settings Generator does not take the Generate Texture from the main window when you open it?

However, I still have two questions now.

The first is about the automatic loading of maps you said last time. I downloaded the terrain file separately, and then I added the script file AutomatorExample according to what you said. When he completes the execution, the terrain file does not generate the corresponding textures. 

The second problem is that when I set the precision, if I do not check the Generate texture option, the downloaded terrain file is very rough and simple. Even if I have adjusted the height quality value to the highest, the downloaded terrain count is very few.
After I tried, I found that Generate texture can't be unchecked in this step. It should be unchecked in the following options after setting. So it is an error or something else?  To be honest, this makes me very confused. This doesn't seem to fit the characteristics of your excellent products. Am I wrong?

Attached are some pictures I have set. Please check them. I look forward to your reply.

You are probably using too high a setting.
Please attach a screenshot of your generation settings so I can check this.
You can also find a pre-calculation of memory usage by pressing the Memory Usage button.

I tried to download terrains by RWT.

However, when RWT generating textures, an error would pop up when the progress reached 85%.

I had tried three times and adjusted the download level, but the result was the same.

Please see the attachment for errors. And I wish you can give some advice. Thanks!