Forward Delete only deletes one character at a time - it would be nice if it would keep deleting until you release the key combination.
you can enable repeat for most BTT triggers. What exactly are you trying to do?
Thanks for the hint about enabling repeat - I found how to do that, and it works. Only improvement would be to hide the mouse pointer/cursor when deleting, as it flashes on and off with each character deleted.
In general Karabiner Elements is better suited for such tasks as it does actually remap the keys and can work on a lower level. BTT is a bit limited on how it can intercept events, but hiding the cursor would be a good idea.