So this is the most cost-efficient skill for tanking (increasing constitution increases HP with the same rate, but defence is more valuable because of logarithmic scale I mentioned in earlier message; increasing perception only gives 2% evasion per 2 skill points).
You are forgetting something very important, and that is that talent points are precious. And those other skills do other things.
Constitution increases HP, it ALSO increases your stamina and reduces the cost of noncombat actions
Perception might be "only" 2% def instead of 2.5% def per rank as you say... but unlike armor it has more going for it. it gives you items, and boosts your damage via crits, and land debuffs.
By the time you have gotten the minimum required skill levels in everything else and can afford to start dumping points into armor, you have already outleveled everything.
So pray tell what exactly are more valuable talent points for Tia? She is a tank and should be able to sink as much damage as possible
Oh, you were talking only about Tia and not the MC and Lyvia.
Tia specifically has fewer things competing for her talent points than the MC...
Tia isn't a tank, she has poor defense and high offense. Tia is a DPS. Tia has the highest damage in the party. Her damage is higher and her defense is lower than Lyvia.
She has a completely useless taunt skill, this does not make her a tank.
Not only wastes actions (by taunting instead of killing the enemy), but also even if you go full tilt into boosting her defense she will still get annihilate by enemies if she tries to use it.
Also, she lacks the agility to cast the taunt skill before the enemies attack and damage the rest of the party. To be an effective tank one needs massive agility so that they can cast their taunt skill before the enemy attacks. Massive HP and massive defense so they can take hits for days.
Tia cannot do any of those things.
Far better to pour in her talent points into strength. Where she gets a huge boost to damage.
Also Tia is very useful in harvesting resources on the map which benefits from a bunch of stats which compete for her talent points