I didn't know DDS was so well optimized for RAM, CPU, and GPU. I didn't know that JPG had such a significant impact during decompression. I won't even mention the mipmaps, as that was far beyond my understanding. I believe that DTX1 and DTX3 are currently the best options available.
From what I read:
DTX1 (BC1) was released in 1998 with DirectX 6.
DTX3 (BC2) was released in 1999 with DirectX 7
After acquiring this knowledge, I experimented with all the newer formats up to BC7 (DXGI_FORMAT_BC7 which was released in 2009 with DX11). However, in my experience, all RGB/RGBA tests resulted in the same disk size or even worse outcomes. Although I read that BC7 offers better compression for alphas, I didn't achieve a smaller disk size. In theory, this compression improvement should come at the expense of CPU time and GPU resources. So, the end result is that I didn't manage to find a better solution.
The only thing that worries me is that this format hasn't improved in a significant way over the years, so we cannot benefit from it, but that's life.
Thanks for clearing this. This is a simple example of how we learn something new every day.