I nominate this question/topic for the first video tutorial in the forum: how to apply a filter to a part of a video clip. I will make a video for this within a couple of days. Meanwhile, the short answer is: apply the Text video filter. The first mini-video-tutorial shows how to add a filter to a clip on the timeline: https://www.shotcutapp.com/tutorials/minitutorials/
The challenge, however, is how do you limit the filter to just a portion of the clip? The short answer is to Split the clip so that you can select just the slice you want when applying the filter. There is a Timeline toolbar button to split a clip or use the ‘S’ keyboard shortcut.