Nit: How come that file name 99defaultrelease? High number yes but maximum is bad since it can never be overruled.
Yes, stretch.
Indeed. Minor: When we make this a wiki template however it may be easier to just add -t buster.
We at Whonix don’t control what stable points to. Debian does. And when Debian does this can break things. Happened in past with electrum and our apt pinning template. So better leave it at specific codename stretch rather than generic codename stable.
If it works why not.