How can I get rid of extra line space in this poetry stanza

Yes custom HTML block can be a solution but I am sure you also need the formating options in block editor.

Here is a simple solution, Just create a css class:

.redspace {
  margin-bottom: 0; //what ever you like the space to be: 10px 5px...
}

Now apply this class to the p where you dont want the space.
in your case:

<p style="text-indent: 25px;" class="redspace">Then the sun sinks, swallowed by the dark. In that dark
more dust, always more dust</p>

Thats it.