I still use CS4. I save everything to a v8 file and make sure before I do that masks and clipping paths are released. I have found that sometime these interfere with the generation of objects. I also change the import scale on the formZ side to a factor of 12. This will bring in vector art at a physical size of feet (meters)
Lately, I've found the DWG is cleaner in formZ v8. However in both cases, you do have to go back through an boolean difference the inset shapes in letters or patterns. Then you can extrude, reshape, etc. The Illustrator method always preserves the different object colors, but formZ v8 does not seem to always import them with HSV or RGB values that match. I'm accustomed to going in and simply rebuilding the material in formZ to my spec, as it still is assigned to the objects that were that color in Illustrator.
For context, this has been left this way since the early days of formZ. I understood that Adobe was not interested in providing code to ADS to make this easier. I keep my old (non subscription) illustrator running just for this purpose and have often asked designers to back save their vector files. (If its a CC file, older Illustrators wont open it. Another Adobe problem)
DWG/DXF is always a good alternative. It would be nice if Affinity and others exported a DWG and stayed current with it. I believe they have the same difficulty with Adobe providing code to be native. DWG vector exports can be very helpful for clean CNC pattern making.