I'm rather out of touch with the latest versions of dtp programs, being retired, but I've had experience with PageMaker, Ventura, InDesign and PagePlus, so I've had a cursory glance at Affinity Publisher and it seems to be that it has a great deal of promise.
I'm glad to see that the 'Place' feature includes text files, although I think it should be able to import Word files, a program which I dislike but which is used by a great many people. The problem with placing rtf files is that paragraph styles don't seem to be imported as well and I always depended a great deal on paragraph styles when I was doing layouts of both books and magazines.
The program which offered the most straightforward use of paragraph files was, in my experience, PageMaker. No matter what word processor I was using, all I had to do was place the name of the paragraph style contained by the greater and lesser than characters – <Title>, <FirstPara>, <Body> and so forth – at the beginning of a paragraph and when PageMaker imported the text file each paragraph automatically adopted the features I had assigned to that style. In addition each style, such as <Body>, was also applied to each following paragraph until a new paragraph style was inserted. All I needed to do was to ensure that I had as 'clean' a word processing document as was possible. When I had to switch to InDesign this feature wasn't included and I had to use the word processor's paragraph style feature and hope that InDesign managed to import each style's features, which it seldom did straightforwardly and, as far as I can remember, always ignored italic and bold text.
I suppose it's too much to ask that Affinity adopt PageMaker's feature for dealing with imported paragraph styles...
