send the signal from rear side of the observer to 5 vertically stacked pistons

I'd suggest the observer power a piston with a column of slime blocks with observers glued face into slime. The observers will fire after being moved, and power the pistons - something along these lines:

I'm on Java so I can't verify if the timing will be good enough - if you won't land right into observer cooldown after retraction, so you might need to adjust timings by powering the piston through some repeater, but generally the idea of column of observers on slime should do the trick.