In searching for an Adobe alternative for 2 small newspapers I found a working example that maintains CMYK color space and preserves accurate black text and artwork. This small example is of one ad using CMYK artwork and CMYK swatches to include accurate black (100% K).
The attachment should tell the story, but basically created a 6-column by 2 inch doc in AFPub. I then copied text from an InDesign 2019 ad of the same size and pasted it into the new AFPub document. In the process, the color settings in the INDD doc for Goss 45u2 carried over with the text and also set the AFPub workspace of Monitor August 2015 for a Goss press. The color swatch and the accurate K swatch also were brought over during the copy and paste process (to include the fonts).
When exporting to a PDF, I created a preset as shown while trying to maintain no color conversion. The results for the AFPub PDF and the INDD PDF were exactly the same with measurements from the Acrobat Output Preview.
This probably is not a solution for everyone, but for this small publication company it seems to fill the bill. The real test will be to have a press test, but the PDF tools all measure the same for both INDD and AfPub. Enlarge the attachment to read the process.