Nothing, I haven't found a good option yet.

We do have the existing bindings to a 2-year old version of basis universal, but I've been looking to replace it.

I don't think you'll find anything much better than basis universal, assuming you want textures compressed on the GPU and the simplicity of shipping one file that decodes quickly enough. I've followed development of the encoder, and its authors know what they're doing.

You might beat basisu if you encode for one texture format at a time, and use perceptual RDO for albedo textures.

Another alternative would be to use JPEG XL for distribution and transcode to GPU texture formats on install, but you'd have to ship a decent GPU texture compressor (fast ones leave quality on the table, and it's really hard to make a good one that isn't exponentially slower).

Is basis universal that bad? I thought it's more or less invented for this purpose.