@OwenNZ
You describe exactly what I do, and for me, it seems to work.
The important thing is that both Body and SubHead must be fully specified **Paragraph** styles
I apply "Body" Style to the empty frame; Body includes a default char style of the desired font settings
Then I import the text - either in Story editor; or Get Text; or a plain text Paste
In my case that gives me all paragraphs with 6pt spacing after, but I want my subheads to be Bold with zero spacing after.
Personally I find working in Item Edit mode easier than in Story editor at this stage
The F3 Text palette shows Body style & [Default Character Style]. Don't touch the lower dropdown.
In item edit mode, place the cursor anywhere in a subhead para, and set F3 paragraph style to SubHead.
Repeat for each subhead para.
For me, this works and formats the selected paragraphs as desired.
I have issues with Get Text, but that's another matter really.
To make the importer work you need to set which importer to use; it doesn't set the importer from the imported filename extension, as it should; but at least it remembers what you did last time.
Hope this helps - either you or any other browsing the page
Spilly