I did reboot, disk utility, as a verification shows /volumes /nix as writable, I ran the installer in daemon mode, seemed like it was going along nice but at the end still got the same r/w error. I noticed it did try to mkdir the /nix disk even though it was already corrected.
sh <(curl https://nixos.org/nix/install) --daemon
sudo mkdir -pv -m 0755 /nix /nix/var /nix/var/log /nix/var/log/nix /nix/var/log/nix/drvs /nix/var/nix /nix/var/nix/db /nix/var/nix/gcroots /nix/var/nix/profiles /nix/var/nix/temproots /nix/var/nix/userpool /nix/var/nix/gcroots/per-user /nix/var/nix/profiles/per-user
to make the basic directory structure of Nix (part 1)
mkdir: /nix: Read-only file system
mkdir: /nix/var: Read-only file system
mkdir: /nix/var/log: Read-only file system
mkdir: /nix/var/log/nix: Read-only file system
mkdir: /nix/var/log/nix/drvs: Read-only file system
mkdir: /nix/var/nix: Read-only file system
mkdir: /nix/var/nix/db: Read-only file system
mkdir: /nix/var/nix/gcroots: Read-only file system
mkdir: /nix/var/nix/profiles: Read-only file system
mkdir: /nix/var/nix/temproots: Read-only file system
mkdir: /nix/var/nix/userpool: Read-only file system
mkdir: /nix/var/nix/gcroots/per-user: Read-only file system
mkdir: /nix/var/nix/profiles/per-user: Read-only file system