Another idea: if the text container is in the same position on all slides, you could consider using the Timing Properties panel for it and set it to display for the rest of the project. But I suspect that the position is not the same on all slides with that text container. You didn’t specify how long the text is, and you need to know that when inserting the variables (using the X button) you may need to set the length to more than the default 50 characters in the Insert dialog box. I include a screenshot of that dialog box, where I changed the length to 256 characters (which is the maximum).

I don’t use the old text captions anymore, always use shapes as text containers but the answer is valid for both.

Create a user variable, I will label it v_content.  Insert that variable in all the text containers using the X button in the Character part of its Properties panel. Then you’ll need only one action to Assign a value to that user variable, and all text containers will be updated. You need to do that before the first text container, you could use the On enter event of the slide where that text container appears.

Beware: you have now ‘dynamic’ text, which means that it is very important to use a websafe or an Adobe font for the text. That is the only way to guarantee that the text generated on runtime will look the same on all systems using your course.