I would suggest editing your .20mm setting down to .16 and then down to .12 to see if your threads come out smoother. Also you might try using the variable level option in studio to see if cleans up a bit, good luck!
I print a lot of threads in my designs. There really are only two secrets.
Use as fine a layer height as you can. 0.08 is a good place to start.
Always print the part vertically and make sure that supports are enabled. I found that changing the spacing from the default of 0.35 to 0.70 leaves a generous gap that make the supports easier to remove. However, if you have a shallow enough slope <45 degrees, the slicer may not generate supports indicating that the slicer predicted that the filament would not need them. You can change that angle in the supports section.
Here is where the angle and XY gap distances may be found.
Notice the two examples here. AT 0.28 layer height the slicer wants to add supports. at 0.08 layer height they aren’t needed because the layers are much finer.
Feel free to play with this M12 bolt and nut. This nut and bolt conforms to the M standard and actually works with metal real world counterparts found in your local hardware store. Nut M12 x 1.75 mm Thread, 12 mm Long example.3mf (689.3 KB)
I used this example to practice designing threads in OnShape to see what would an would not work. I can say that even when I had perfect print, I still had to cut the threads either with another plastic printed part or with an actual metal tap or metal nut. A few turns back and forth really made the difference but be careful of the tolerance. If your bolt is too fat, it will break off. In that case change the contour XY tolerance to slightly smaller.
One last tip, after you’ve work the two parts together with a few sample turns, a dab of PTFE lube, the same lube you use on the printer, is the final step of making smooth threads. But do this last as you want the threads to cut each other first to wear off the hot spots, then add the lube.
