You shouldn’t really make an image something with that high of a resolution, try going for something below 1024, as for the image I think making the text size more consistent would be better, and change the color palette a bit.
There isn’t much you can do when it comes to quality with exporting your UI designs. What you can try to do is whatever external program/software you used to make your UI, you should ungroup the whole thing and just export piece by piece, and then assemble everything to how you want it. Also, do not export your text. The quality is not very nice when you export text by itself and not grouped.
Agree to this, if you want to maintain the closest in terms of quality, you should scale your UI dimensions based on the dimensions of your image. This can be easily done using the offset property of Size within the ImageLabel.
Adding on to that, make sure you set ScaleType to Fit instead of Stretch. Adding a UIAspectRatioConstraint can also help.
My mind initially went to exactly what you said. It seems like an odd thing to use an image for: Something filled with text. You could make frames and backgrounds and text labels and have this look crystal clear, and configure exactly as you want it. Use images for things like the envelope and other small icons.
Essentially, the impractical route would require you to crop out all the elements in photoshop one by one and increase the quality in each element by following the tutorial. Again, VERY impractical, but if it’s a desperate call, then try it out and let me know how it works!