Yes they should serve their own purpose to the faction, otherwise they wouldnt be necessary^^
But yes i like shagrats idea, i think the mordor trolls should have lower durability with higher dmg output with their weapon specialisation
and the Dol Guldur trolls have higher durability with their increased armor, but have lower dmg due to the fact they only get the wooden mace, but they could be used more overall.
And how and from where you get the Dol Guldur trolls:
I still think they should be recruited from Dol Guldur, and you would get them through Saurons influence. This would symbolise the fact that Dol Guldur which is a fortress far away has their own unique looking army. And with Saurons prescence, he armors up his army with heavily armored battle trolls.
And i dont think Minas Morgul should have trolls as a unit, because Minas Morgul is a fortress in Mordor and all the trolls there have the same armor and Dol Guldur has its own.
But it could also be through the troll cage:
Maybe through Saurons influence, instead of giving the trolls an upgrade he would give the troll cage new units, Dol Guldur battle troll, and Mordor attack troll.
So heres the idea:
Troll cage level 1
Mountain troll: These would be trolls in training, cant get any weapon or armor upgrade, but they are cheaper, and mostly used in the beginning. Ofc they could be used later if youre running low on resources.
Troll cage level 2
Drummer troll: same as now
Troll cage level 3
The trolls spawn with armor.
Mordor attack troll: You get the three weapon upgrades for largely increased dmg, but the trolls have lower durability, in the movies it was always these trolls that were used to breach gates and they did as much dmg as they could before dying.
Dol Guldur battle troll: Heavier armor, but no weapon upgrade, they spawn with the Wooden mace, and dont have as high dmg as the attack trolls. Their dmg would be more overall due to the fact they dont have weapon specialisation. And in the movie these trolls were fighting on the battlefield for a longer period of time.
These would serve you as tank trolls, when you are fighting a battle and you dont have that much time to focus on you trolls, while Mordor trolls require more attention to be used effectively