Thralls do not have stamina, and giving them stamina is not really a good idea. However, there is an imbalance that arises from their infinite stamina, allowing them to use weapons that have stamina cost multipliers with no penalty, or, in the case of the Sword of Crom, are prevented from using it at all because it would be too powerful in their hands.
The solution I propose is what I call a “Stamina Rating” for thralls.
A Stamina Rating is not stamina, as it would not deplete, but be a static number for each thrall.
How it might work is every thrall starts with a Stamina Rating of 0. Then they can receive initial bonuses depending on their faction and tier. Perhaps something like +2.5 if they are in the Medium experience category and +5 if they are in Hard. +1 if they are T2, +3 if they are T3, and +5 if they are T4. Thus, a T1 in the Easy training category would start with just the base 0 Stamina Rating while a T4 in the Hard training category would start with a Stamina Rating of 10.
For every point in Grit, their Stamina Rating could increase by +1.5 and every point in Agility could gain them +0.5.
So, what would the Stamina Rating actually do? It would weight the thrall’s attack decisions. A thrall with a very low Stamina Rating would be more likely to do light attacks rather than heavy and fail to complete combos. The higher the Stamina Rating, heavy attacks would be weighted more and the thrall would be more likely to follow through with combos.
This would only be a weighting, a thrall with 0 Stamina Rating could still do heavy attacks and full combos, but it would be less common. I think a Stamina Rating of 30 or so (Hard T4 with 10 Grit and 10 Agility will be at 30) could be a target where the weighting for a light attack and heavy attack are 50/50 - below 30 favors light attacks, above 30 favors heavy.
If a thrall is given a weapon with a stamina cost multiplier, this multiplier would penalize the thrall’s Stamina Rating. E.g. a thrall with a normal weapon and a Stamina Rating of 30 would have the same attack weighting as a thrall with a 1.5x stamina cost weapon and a Stamina Rating of 45.
This idea would add an inherent benefit to late-game and higher-tier thralls, increase the value of Grit and Agility, and, of course, make weapons like Momentum and even the Sword of Crom more balanced in the hands of a thrall.