2 things bothering me

[Rykon-V73]

I tried to reduce the attack damage of the Cannon by 8, using:
Attack Damage Loss Bonus, but that didn't do anything. Also, I tried increasing its armor by 3, but that is used by other upgrades. Is there a way to make the tower increase its armor by 3 only with this upgrade and a way to reduce its attack damage by 8?
I don't know the actions, but the rest I know:
Events
Unit - A unit finishes Research
Condition
(Researched Tech-Type) Equal To Building Toughness
Action
...

Some help would be nice.

 

[A]mun]

I'm pretty sure you don't need triggers for this.
Head to your wc3editor -> File -> Preferences... -> General -> tick on "Allows negative real values in the object editor"

Now you can change "attack damage bonus" to negative values.

 

[defskull]

Or even faster, hold down your SHIFT key + Double-click on the field - it allows you to input negative values.