I really, really like NuShell. The realization that maybe I had to leave was, I'll admit, mildly crushing. And then I just had the realization that maybe I don't have to leave NuShell and slink back to the dirty, byte infested corners of Bash and Zsh. Here's what just happened:

/Users/sjohnson/Sync/coding/flow_analytics/open_source〉rails new pool_api --api                                                            
Rails is not currently installed on this system. To get the latest version, simply type:

    $ sudo gem install rails

You can then rerun your "rails" command.
/Users/sjohnson/Sync/coding/flow_analytics/open_source〉ruby -v                                                                                 07/20/2022 05:34:41 AM
ruby 2.6.8p205 (2021-07-07 revision 67951) [universal.x86_64-darwin21]
/Users/sjohnson/Sync/coding/flow_analytics/open_source〉rvm use        

And that was the point when I realized "Oh yeah – RVM doesn't work on NuShell" and then it struck me – I can run Bash:

/Users/sjohnson/Sync/coding/flow_analytics/open_source〉/bin/bash                                                                               07/20/2022 05:34:57 AM
bash: /Users/sjohnson/Library/Python/2.7/bin/powerline-config: /usr/bin/python: bad interpreter: No such file or directory
bash: /Users/sjohnson/Library/Python/2.7/bin/powerline-config: /usr/bin/python: bad interpreter: No such file or directory
bash: /Users/sjohnson/.iterm2_shell_integration.nu: No such file or directory
ERROR:  Can't find Ruby library file or shared library lunchy
usage: dirname string [...]

The default interactive shell is now zsh.
To update your account to use zsh, please run `chsh -s /bin/zsh`.
For more details, please visit https://support.apple.com/kb/HT208050.
[sjohnson:~/Sync/coding/flow_analytics/open_source] [base] $ rails new pool_api --api

create  README.md
create  Rakefile
create  .ruby-version
create  config.ru
create  .gitignore
create  .gitattributes
create  Gemfile
   run  git init from "."

So maybe my system can mostly run NuShell and I can drop in and out of Bash / Zsh when I need different things. Fingers Crossed.