Hi!
After playing insane amounts of Battle Brothers hours (its the only thing I play really :p) I really believe that after ANY 3 category I perks, ANY second tier tree could be followed, and then ANY third tier tree could be followed after 2 more in that category. This would create a greater diversity of characters, and allow for more balanced rather than specialized top-tier characters… not to mention that some of the perks are actually in the wrong category ;p