I had similar issues Importing my designs from affinity designer into sheetcam.
After all was said and done, I made sure all the units were selected the same, then imported a 10 inch square. I then measured the square in sheetcam and it was 10.6667. I divided 10 by 10.6667 and got .9375. Now I scale everything I import into sheetcam by .9375 and it’s all perfect.
DXF was more trouble that it was worth for my 2D designs. SVG is much easier and the format works with the programs I want to use.
There is probably a version, unit or scaling option you may be missing, or you could see if the math works out for you the way it did for me. If you’re already using sheet cam, you can ditch fusion for an svg editing program.