Only got 1 thrall with the acc perk at lvl10. He had only +2 str gear on that time (old silent legion with godbreaker boots).
My current Spinas at Lvl10:
Always in full str gear which gives +10 Str (besides godbreaker boots) and at Lvl16:
Leveled 2* Cims and 1 Dalinsia to lvl20 in full str gear and no acc perk.
2 other Dacias at Lvl10-13 in full str gear have the +2 all perk.
Also which thralls did you level as their starting attribute points depends on faction?
PS.: My thralls which I level up are using Ajas bane which gives another +5 str.