I consider PVRTexTool to be pretty much the best tool I tried so far in terms of KTX2 support. The only thing I find to be missing is support for BC6H, R8_SRGB and R8G8_SRGB. Especially BC6H is pretty popular for HDR content, so it would be great to have it supported.
I also noticed a small mismatch in that exporting a BC1_RGB_UNORM_BLOCK texture again will turn it into BC1_RGBA. Since this is an extra nuance to Vulkan it would be nice if BC1_RGB could be officially supported too for export.
Thank you for the great tool!