I'm trying to find a good workflow for adding subtitles to a video.

I have a text document that contains the subtitles (in this case, the lyrics of a song).

My intention was to cut/paste the contents of the text document into the subtitles edit window, and then use the scissors to split the first line off the rest, adjust the duration, and then continue with the next subtitle line. While this works, it is a bit tedious since the scissors only work when the playhead is over the subtitle. So when I listen to the video and get to the point where I want a subtitle to start, I must let the payhead advance a bit and move the subtitles to just before the start of the playhead for the scissors to work.

So I was wondering -- is there a better way to add a bunch of subtitles starting with a text document?

No, not at the moment. What I'm doing is I cut and paste words from 1 subtitile to anther to make finale adjustments.
Another way is to use other Software generating the SRT file from the text you have.
Or you put the whole text into 1 subtitle. Make 1 or 2 cuts. Then export the SRT subtitle and make the cut by hand in an editor and re-import the SRT file into Kdenlive.