Your software cannot tell how many combo points you have, or how much rage you have.
It cannot tell if an ability actually hit, or was fired off but blocked or missed.

So it depends on how much you want to manage yourself.
Versus how much you want to occur as you have preconfigured, which won't always be optimal but will allow you to manage other things instead.

You can make a mapped key for Hamstring.
You could give it three steps.
Step 1: Hamstring Keybind, Do Not Advance for 2.0 seconds.
Step 2: Hamstring Keybind, Do Not Advance for 2.0 seconds.
Step 3: Nothing, Do Not Advance for 16.0 seconds.

If you were to insert that into your spam key.
It would attempt to Hamstring for 2 seconds as you spammed away.
It might fire off a Hamstring or something else might be fired off first.
Once you have been spamming for at least 2 seconds it advances to the next step.
After four seconds of spamming, whether it hits or not, it won't try again for 16 seconds.
If you positioned this early out of your actions, it is more likely to fire off than if it is after a bunch of actions.



I assume for Rend, you want it to fire off once.. whenever it is not up.
And to then not fire off again, until the duration has expired.
The software cannot tell if it has landed or not.
Just whether the keybind was sent or not, and how recently.

You could get a complicated mapped key that might work.
Probably with a toggle function.
Have a mapped key for Rend.
And have that as part of your spam set up.
Either off or on, by default.
Then build a toggle key, which you manually press to enable/disable the Rend mapped key.
When it is enabled, it can fire off.
When it is disabled, it cannot fire off... which means you cannot blow rage on another Rend when it is already ticking away.

It depends how important precise control is, and not wasting resources.
Versus how much you want to preconfigure, which may not be optimal but might be sort of close.