Interesting stuff Los, I will need to give it a full read over. I have found this summary by CharlesDM is super useful.
http://www.bythisaxe.co/2015/07/acks-ability-scores.html?m=1
(Very inspired by his material btw and am using a derivative of his tinmen in my home campaign)
The connection between intelligence and wisdom is strong, I wonder if we find it easier to separate physical characteristics as we are better able to describe a feature. In hindsight Gullible in wisdom is INT related, swap it with unobservant below.
I found a common interpretation of low intelligence is mental illness, which caused me to recheck my constitution negatives. Low stats may indicate a susceptibility to a thing, the same as constitution, but it is not a condition itself.
I have focused on Intelligence as the ability to learn and apply knowledge.
Things like the feeblemind spell give good clues. And I’m trying not to penalise survival or navigation as it impacts the party, not just the PC.
Next, back to the beginning, strength.
Intelligence
The character does not suffer the standard intelligence penalty and instead selects a flaw from the following list for each point of intelligence penalty. If not custom selected the character is assumed to have taken illiterate for the first point and incoherent for any remaining points. The intelligence penalty is still used in the strategic ability score from domains of war.
Illiterate (default): the character is unable to read or write. When confronted with text in any language they cannot interpret the meaning. They may not take the knowledge proficiency.
Incoherent (default): the characters use of language is poor and they suffer from a low vocabulary. They receive a cumulative -1 penalty to reaction tests when using language. Can be taken multiple times.
Uneducated: the character lacks formal training, be it from a school or apprenticeship. Each time this is taken they receive a cumulative -1 penalty on proficiency throws and thief skills. They cannot take the the profession proficiency. May be taken multiple times.
(Can be partially mitigated through proficiency choices but may still be used by the judge)
Slow learner: the character is not the sharpest tool in the shed. They receive a -5% experience point penalty penalty. This penalty is subtracted from a prime requisite bonus if they have one. The character cannot take the loremaster proficiency or a class that possesses it.
(Would you take it it you had 10%?)
Fearful: the character has learned that sometimes being scared and running away is more likely to keep them alive. They suffer -4 to saving throws verses fear. They cannot take classes or proficiencies that make them immune or resistant to fear.
Swap Gullible in wisdom to INT related, add to wisdom:
Unobservant: the character often misses small details focusing more on the general environment. Whenever they suffer from cover or blindness penalties they increase them by +2. The character may not take the blind fighting proficiency.