I tried to follow the same steps but the install_cmdstan script never terminates (with cmdstanpy 0.9.68).
With the --verbose option, it is stuck at (probably waiting for something, not consuming any CPU):
make[1]: Entering directory '.../.cmdstan/cmdstan-2.26.1/stan/lib/stan_math/lib/tbb'
However, I was able to reinstall cmdstan directly from source and run models with cmdstanpy.
The only thing I add to do was to comment out the following line in make/stanc.
xattr -d com.apple.quarantine bin/stanc