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In general, here's what didn't work out. I couldn't make it 1920x1080.
I can create an image, or I can set parameters for this image to that size. BUT!!! the entire UI is somehow built on 1024x1024. If I try to edit it for a larger resolution, it just shifts out of place.
So, 1024x1024 is what's possible. Again, in reality, the values are 1024x768 (that is, 1024 in width and 768 in height) everywhere.
The rest is usually taken up by the map description.

So, what do we have in the end? We can make the center 1024x1024. The sides will be standard, like the other screens. This isn't because I'm doing something special; it's because the other screens (from the first version and the Hammers of Fate expansion) are also built this way. And the sides are cleverly done: they use a stretchable texture that is 1024 pixels high and 2 pixels wide. And everything extra on the edges is simply cropped by a duplicated texture (if I understand correctly).
For now, it looks like this:

P.S. Did you guys know that there's no icon for Absolute Training for the Knight in the game?)
Added after 11 hours 3 minutes
Result:
Added after 23 hours 51 minutes
Is it possible to somehow influence the rune settings? For example, reduce their effectiveness?

No need to talk about how you can go into the xdb and edit the runes; I know that. But there, you can only change the rune level and the resource cost.