Being as thralls get way less than 2% dmg bonus per STR (I think its something like .04% or something silly). STR doesn’t really play a role in dmg very much. Thralls have a hidden stat that increases their damage by a multiplier, its anywhere from 1.105 to 2.24. So a 100dmg weapon will do 110-111 with the weakest T4 thrall and 224 damage with the strongest. This is mostly dependent on the tribe they come from. Siptah plays with these rules a bit, but there’s videos about that.
Agility is problematic too as it is also based on tribe. Some get like 8 armor per AGI, and some get 12 or 16. But like someone already said, heavy armor makes this kinda moot unless your thrall is constantly fighting heavy hitters (usually only applicable in modded games).
Vitality like agility varies from tribe to tribe. They can get anywhere from 40-160hp per VIT. Sometimes the lesser damage thralls get more HP, but not always (usually a Siptah thing). If your thrall can solo stuff than VIT isn’t needed. If they can’t then consider some VIT gear.
Survival is a great stat, especially with anything with dots. A high enough SUR stat will cut a dot time below half, which means the damage taken is less than half. It also applies to cripples and sunders.
So here’s my recommendation (assuming T4 thralls). If you’re playing unmodded PVE, then you can probably focus STR. Its not much but you likely won’t need the other stats so more damage means you’ll generally kill faster. It won’t be much but it will save time over a long time.
If you’re encountering alot of DoTs, then focus survival. It will be less of a headache overall.
If you’re playing modded content like Age of Calamitous or Extended Engame Weapon Arsenal then VIT and even AGI might be a great option. So diminishing returns on armor is a bit of a misconception.
So once you get higher, yes… it takes more and more to get the percentage. But you’re looking at the situation backwards. For example 70% dmg reduction going to 75% means the thrall is taking 17% less damage. A 100dmg hit hits a 70% armored character for 30dmg, and a 75% armored one for 25dmg.
So even though it looks like armor has diminishing returns, the TTK scales a bit linearly.
But this isn’t needed on normal content where the highest hits you take are in the mid 3 digits. When you play with AOC or EEWA then you start seeing 1000-2500dmg hits that need to be mitigated as much as possible.
In PVP it probably helps to focus on VIT and SUR to ensure survivability. But this is going to change on a per server basis, and if official or not.