I did a tutorial on how create an Optimized Config for Fortnite Battle Royale on XIM Manager
It has a working Auxiliary Build Mode sub-config, where you can use a different sensitivity or keys for your wall, stairs, floors and roofs.
I had be using this setup for months. If you want to check it out:
LinkIt has Configs for XIM4 and XIM Apex.
Just change the keybinds for your favorites and follow the instructions.
Very handy, I use a curve for build singles with a short mouse swipe.