I use Krita to paint textures for 3D models a lot, and I am really fighting to transform images to match the UV layout. I was hoping for the Warp transform function to allow me to modify an image by adjusting vertices of the grid, distributing the image equaly. But it modifies pixels nearer to the moving vertice more than pixels further away. Maybe the name should have been a warning, it really is warping the image.

Might come in handy another time, but I am still looking to transform the image by a grid in a more predictable way, just like a 2D texturing. Does anyone know if there is a setting, or even a plugin, to do this?

Have you tried the Cage transform, to the right of the Warp transform on the Transform Tool Options docker?

If you click around your image to make many control points and then finally click back onto the first control point, to define a closed region, you can then drag the control points to give finely controlled 'stretching' movement of the image inside the region. This may be suitable for you.
To finish, select another tool, e.g the Freehand Brush and the stretching you've done will be fixed in the image.

In the Brush Presets docker, the Move Tool brush and the Distort Move brush can give fine local adjustment of image position.