While mature and time-honored, not modern, utilities like sed and envsubst go a long way to managing any config. Making a "modern cli" application that wraps existing functions is like giving someone a fish, as opposed to teaching them to fish with stable and ubiquitously available tools.

Create entry (using a template and exported variables):

  $ envsubst < .ssh/template.config >> .ssh/config

Read entry (assuming the 4 line template):

  $ sed -n /<Host entry>/,+3p .ssh/config

Update entry: Delete and Create

Delete entry (assuming the 4 line template):

  $ sed -i /<Host entry>/,+3d .ssh/config