Yes, it is suggested to use Note Spacing Changes (in Engrave mode): set first a Reset NSP on the next bar after the text box, and one at the beginning of the bar with the text box.
It is better to decide how to cast off the text+music, and then lock the layout (or just the frame where you are working) to avoid the NSC to rearrange the layout.
Using Keyboard Maestro and Stream Deck I made some macros to insert various NSC values, as well as a Reset NSP, so that I can insert them very fast. And you can also Option(Alt)+clik them to copy them in other places (in Write mode)
Also, to make the boxes cover the bar rests, staff line and the fermatas, set the background (for the Narrator text customised Paragraph Style) to white, don’t use the Border functionality of the Paragraph style, but the Border Property in Properties panel. When I have time I make a video Explaining.
Here an Example of my implementation of Narrator text (for long text it requires some manual vertical adjustment, and sometimes a manual staff spacing adjustment, as currently the text position, and its avoids collision, can be only be Above or Below the staff, and not centered “inside the staff”):
Dorico file example (edit: updated with Narrator layout with cues, and narr. text cues in the flute part):
demo narrator.dorico (1.8 MB)
And here my keyboard maestro macros (for NSC reset, NSC 8, NSC 12, NSC 14, NSC 20). You can assign them to Stream Deck buttons, using the keyboard maestro stream deck integration (KM link). For them to work, you need to set in Dorico the Key Command for Note Spacing Change to Shift+Control+Option+Command+N.
cgr macros for dorico NSC.kmmacros.zip (152.7 KB)
Here a video (sorry, it is 6:37 long) that shows the usage ion the macros to insert the Note Spacing Changes, and a possible workflow on how to treat the narrator text:
@lim.usic
And here the promised (bonus) video, where I show a possible workflow to create a Narrator layout with cues 
:
Result of Narrator layout with cues:
Here finally a video that show how to use the Narrator text to create cues (also with only partial chunks of the original text) for the parts, using the Start text functionality of cues:
Result:
If you need to have different line breaks in the Start text field, I use keyboard maestro with an Insert Line Feed macro:
