Hi Jelle.
Welcome to our Mattress Forum. 
Sorry to hear the zoning in the Avocado is causing pains for you both.
This is a great starting point. Like you mentioned, it’s possible another layer may be needed depending on how much you sink into the 3" of firm latex. As for the 6 vs 8" coils…
The height of a pocket coil is just one of many variables that will affect the feel and performance of the coil and how it responds to pressure and how well it can contour to the shape of the body. Some of the others include the gauge of the coils, the number of coils and the coil diameter, the number of turns in the coil, the shape of the coil (cylinder, barrel, etc), and the specific design of the coil (single spring rate, multiple spring rate, etc). Some mattresses that contain taller coils that can contour to the shape of the body more effectively may need less padding materials above the coil as well. That said, innerspring/coils are not normally the weak link in a mattress, it’s usually the comfort layers that cause the most issues.
There is also more about the pros and cons of different types of innersprings in this article and in post #10 here.
That said, 8" coils tend to be the preference for most people (and I believe they are slightly more firm), but aren’t strictly necessary to a successful firm set up
NikkiTMU