I see, thanks a lot for the explanation.
adding the ability to specify a custom certificate path
It's already supported by Core Tunnel, you can set CertificateFile directive:
appending the custom command prior to the diagnostic command
custom-jumphost-cli is a CLI tool, and Core Tunnel has no idea about its internal. Even Core Tunnel could run custom-jumphost-cli installCert before actual connecting, but it still has no means to interactive with the cli command.