So, couple things. First, in the Klipper macro, use two equal signs:
{% if INIT_EXTR == 0 %}
That’s why you’re getting the error and will probably solve your problem.
But just in case…
Second, I’ve got a 6-in-1-out setup (so, different from yours) but this may be helpful. My SuperSlicer Start G-code block sends a PRINT_START macro with a CURRENT_EXTRUDER parameter, and that parameter is defined like this:
CURRENT_EXTRUDER={if current_extruder == "0"}extruder{elsif current_extruder == "1"}extruder1{elsif current_extruder == "2"}extruder2{elsif current_extruder == "3"}extruder3{elsif current_extruder == "4"}extruder4{elsif current_extruder == "5"}extruder5{endif}
I know that can probably be simplified but it works so I don’t mess with it.