After the above error messages I decided my python installation was cursed.

Following this answer, more or less: https://apple.stackexchange.com/questions/284824/remove-and-reinstall-python-on-mac-can-i-trust-these-old-references

I did

brew uninstall --ignore-dependencies --force python
and
brew uninstall --ignore-dependencies --force [email protected]
and

brew uninstall --ignore-dependencies --force python@3

and then just

brew install python

Now I can make virtual environments just fine.