Just as a blacksmith refines his skill and the results of his work become less random, the more you temper items the more you get closer to refine your gear less randomly.
Tempering Level 1: As it is now
Tempering Level 2: ability to choose the affix, but the percentage roll remains random.
Tempering level 3 is mastery: ability to choose the affix, roll is always between 70%-100% of its max value.
Make the xp progression through these three tempering levels of mastery thorough, not easily reachable but not overly long either.
Tempering mastery is unlocked for your character, not account-wide (tempering from level 2 or 3 binds the item to your character).
I don’t know if it’s a great idea or not, but it would give it a natural flow since it’s based on the fundamental way to increase your skill at anything in life, would feel satisfying to me.
(I have another idea on top of that but this might be bad / not fitting: could add the skill check system from Dead by Daylight as a way to boost xp for your tempering mastery)