The Umami demo: Content module is not found from Drush, nor is it visible in the list of modules, because it is set as hidden in demo_umami_content.info.yml. If you change the hidden: true line in hidden: false, you will be able to uninstall the module, and re-install it. This will execute again demo_umami_content_install(), which imports the content.
Ah 🤦♂️. Trying to install my rutherford theme via the web UI gave a The rutherford theme was not found. error message, so it seemed the theme was at fault, not the profile.
I inspected my theme's info file and realised it had a dependency which was not being met. Commenting out the dependency got everything working again!
It's a shame that the php www/...
1) learn about drush cex (drush config-export)
2) when you make config changes run drush cex then commit the changes.
3) on live when updated form git you need to run drush cmi (which imports the config)
4) stuff to watch out for you ...