Hey man, I just ran into this mod here that allows a class to have up to 7 PERKS per rank. (or you can just stick to 3 skill-tree like in Long War). Seems all you gotta do is add more abilities to your skilltree per ranks. (any user without this expanded skilltree mod would only see 2 skill per ranks as usual.)

Check it out man!
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=621327373