Note that KTX2 is a container format. It can contain uncompressed or compressed texture data. Some of the tools you mention might support "KTX2" but they won't compress textures for you, they just put the uncompressed data in a KTX2 container.
And my out-of-the-box Gimp 3.0.4. doesn't have KTX export at all. Didn't check the other tools you mention.
Then you’re missing some plugins that were installed by default in mine.