Solved it, for more info go to Error reinstalling Nix on MacOS (error: failed to configure synthetic.conf) · Issue #8771 · NixOS/nix · GitHub
However, the solution was fixing a dead symlink as stated in Problems with multi-user (re)installation of nix on macOS · Issue #3261 · NixOS/nix · GitHub
Check for an old symlink like this:
ls -la /etc/ssl/certs/ca-certificates.crt
If you have it (e.g. pointing to /etc/static/ssl/certs/ca-certificates.crt, remove and create a new one.
TL;DR: Try this
sudo rm /etc/ssl/certs/ca-certificates.crt
sudo ln -s /nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt /etc/ssl/certs/ca-certificates.crt
Thank you @abathur and everyone else for the help.