Grouping a pre-formatted Picture Frame and a Text Box and then storing them as an Asset is not a bad idea as a workaround, but it can get muddled when you need to change the style, drop shadow etc of the caption.
I am attempting to layout a 40-page book with over a hundred photos, each with a caption. What I really am missing in Affinity Publisher are easy-to-use Captions on Picture Frames. This is a basic feature in Apple Pages and it was a bit shocking that it is missing in the Affinity product, especially when I see that people have been asking for this for a while on this forum.
I realize that it is possible to build captions, but it requires many steps:
draw a Picture Frame,
draw a text block,
align it to the Picture Frame,
set the Text Style.
Group them,
set the Text Wrap.
store as an Asset
That is a lot of steps, for something that is a check box in Apple Pages. Once I had gone to the effort of creating a number of these Picture Frame/Caption text boxes, I encountered another issue, if I wanted to go back and add an FX to the Picture Frame, that is NOT applied to the Caption Text box, I have to ungroup them and start all over again. For example, I wanted to add a Drop Shadow to my Picture Frames. The drop shadow was also applied to the text frame and I could not see an easy way to get rid of it. As mentioned, I am working on a project with over a hundred photos, so you can see why all this would get very tedious to add and group the captions for each photo.
Apple Pages - One Click, enter text. Format frame, without formatting the text.