Can you clarify what you mean by separate system installs?

Today, I manage python via pyenv (local and global state) that let's me create infinite .venvs with different python versions.

Conda does the same job as Pyenv here, with the additional ability to install system-level libraries, compiler toolchains, etc.

i use venv this way. i download and compile specific python versions and install them in a non-system dir with all the other versions. then just run the specific binary to create a venv and it seems to work as expected.