Toggle behavior to turn macro on/off

[checkybox]

So, I want to make the following macro:

when mouse side button is pressed:
repeat BTN_LEFT indefinitely with a configured delay (50ms)

when mouse side button is pressed again:
stop the macro

The closest thing I was able to made is this:
hold(key(BTN_LEFT).w(50))
It works how I want to, repeats BTN_LEFT with a 50ms delay, but only when the activation key is held, and I would like to make it act as a toggle. Is it technically possible?

[KnzHz]

This is also what I'm trying to do ;)