[Jorin Guderian]

Yes, pvrtextool is the best tool to create ktx files. But ktx is just the container format. As internal format please use etc2 rgb or etc2 rgba depending on your image texture. In Addition to that you need to set the check mark to mirror on the vertical axis and the used colorspace needs to be set to linear, not srgb. Compression can be done in fast mode or slower if you have time ��. You might need to create some testimages first and test them ingame to make sure you used the right settings.
But be aware that you do not need to create ktx textures for custom parks to work. Webp files will continue to work just fine. The whole point of using ktx is just to decrease gpu memory usage at the cost of slightly bigger image filesizes on disk. Another advantage of ktx etc2 files is that loading is much faster since the images do not need to get uncompressed as long as you are not on a mac or if your gpu doesn't support the etc2 texture format.