FYI, i managed to complete the workaround attempted above and install a working python3.8 venv in "sid" by downloading and unpacking python3-distutils from "buster"[1] (which is for Python 3.7.3-1: all) and then moving it over in 3.8 directory:
sudo dpkg --unpack ~/Downloads/python3-distutils_3.7.3-1_all.deb
sudo rm -rf /usr/lib/python3.8/distutils
sudo mv /usr/lib/python3.{7,8}/distutils
[1]: https://packages.debian.org/buster/python3-distutils
