OK I WAS GOING TO PUT BODILY-KINESTHETIC FOR SINCLAIR BUT I HAD USED IT FOR SO MANY OF THE SINNERS I WAS STARTING TO BE LIKE "this is starting to feel like a cop-out" but so many of the sinners are so bodily-kinesthetic based!
Also eeeeehhhhhh I mean. Okay. Gregor is easygoing for the most part but he's not really that good with people, he's good with... It's hard to explain. I'm hesitant to say interpersonal because Gregor isn't actually that good at solving conflict he's very much a fawner and he often tries to derail with sarcasm or cynicism, which is... kind of antithetical to having interpersonal intelligence, I feel like? I also think he's been quite a lot more cynical recently in comparison to the earlier cantos but that could just be recency bias I suppose. I'd have to reread every other canto. But imo they've really been leaning especially hard on the "[Gregor] sometimes uses cynical language" part of his sinner description for a little while LOL

Hell yeah! No confusion, I just wanted to clarify that was what you wanted :)

I think for street smarts you have to extrapolate based on hints to an extent, but here are my thoughts on which sinners are the most street smart.

1. Rodya. Obvious. Rodya spent her whole life surviving in the Backstreets, Rodya has the best street smarts, no contest. Demonstrates it too! Her survival intelligence is incredible, only brought down by the fact that she's too reckless, but Rodya is for the most part the only sinner who has provably had to navigate her way out of every possible survival scenario, including as a child, so I don't think anyone could feasibly beat her in this aspect
2. Heathcliff. Also obvious. Heathcliff's upbringing despite being in a rich home + being in a syndicate. Heathcliff spent a shorter time in the Backstreets than Rodya, who lived there her whole life, but he was still an orphan for a short time and then as an adult went back out into syndicate life.
3. Ryoshu. Also, I feel like, very obvious. Ryoshu clearly has background in the fingers, you'd need to be quick-witted and street smart to survive even if you weren't, like, starving
4. Ishmael and Outis. Tied. Both very good at survival, Ishmael probably a bit more but Outis is very intelligent in this aspect. Outis and Ishmael both seem extremely sheltered in terms of Backstreets living in comparison to the people above them, particularly in comparison to Rodya and Heathcliff (due to Outis' comments to Rodya, it can be assumed that Outis has never been in a genuine starvation scenario--Rodya also refers to Outis as a rich kid with a cushy home, so she's probably quite sheltered, but also is obviously hardened from the military) but they've been through plenty enough for me to rank them as having impressive survival smarts. Ishmael's gut instincts are really well developed.

Moderate to little street smarts: Gregor (much more sheltered experience in the military than Outis)
Little to no street smarts: Donqui (reckless, her justice thing is antithetical to survival), Sinclair, Hong Lu, Yi Sang, Faust, Dante (all no experience with the concept, presumably)
Unsure: Meursault (I really haven't seen any indication of how street smart he is because I don't know how sheltered Meursault was, but he just seems like he'd be good at it.)

Going mainly off of memory here, but it's a fun little theory I learned about in high school AP psychology that propositions there are 9 types of intelligence rather than just a general concept of intelligence, and people have various strong intelligences while they may struggle in others! Originally the theory proposed 6 I believe but now it's up to 9. The types are:
1. Spatial-visual (thinking in pictures/charts, visualizing, artistic intelligence)
2. Verbal-linguistic (verbose, good writers and speakers, linguistic intelligence)
3. Logical-mathematical (pattern recognition, equations, scientific/mathematical intelligence)
4. Bodily-kinesthetic (motor skills and coordination, hands-on activities, dexterous, body intelligence)
5. Musical (tone recognition, talent for understanding musical structure, musical memory)
6. Interpersonal ("people smart", conflict resolution, communication)
7. Intrapersonal (highly self-aware, introspective, informed on their own reactions and motivations)
8. Existential (awareness of situations, theory, existentialism obviously, philosophy)
9. Naturalistic (natural pattern recognition, also hands-on activities, categorization)

That being said! I'll pick at least one but ideally two strong intelligences for each sinner.

Yi Sang: Logical-mathematical and existential
Faust: Logical-mathematical, probably intrapersonal
Don Quixote: Bodily-kinesthetic
Ryoshu: Spatial-visual
Meursault: Verbal-linguistic (because he's good at memorizing information and informing), intrapersonal
Hong Lu: Interpersonal, spatial-visual
Heathcliff: Bodily-kinesthetic
Ishmael: Naturalistic, verbal-linguistic (also good at memorizing and communicating information, as we've seen a couple times)
Rodion: Bodily-kinesthetic, interpersonal
(Bonus- Sonya: Verbal-linguistic, existential)
Sinclair: ... is difficult. Spatial-visual I think.
Outis: Verbal-linguistic, bodily-kinesthetic
Gregor: ALSO hard. Sorry. I'm really not sure I have one that stands out for him. I'd say intrapersonal but Gregor is more like someone who thinks he's self-aware when he genuinely really isn't and bodily-kinesthetic I feel like does not apply to him (and is also kind of a copout at this point)

This is a really high compliment thank you anon :') It's not an IQ thing, I had my IQ tested along with everything else when I was 13 and was in testing for autism but IQ is notoriously unreliable. IQ itself doesn't really mean anything.
Reading is a good idea! Reading a lot will help you structure your sentences more easily because it will improve your vocabulary. I've always been naturally very drawn to writing and analysis, but I've also been an avid reader my entire life. I think reading is a very good way to beef up your vocabulary and study how other people structure sentences (this helps with writing in general too, you can't improve your writing if you don't read). I also write essays and read research papers for fun but if you have to force yourself to do that, I wouldn't overwork yourself (I do it because I think it's fun and good practice). I can't say I recommend any specific books, because I've never read a book that's touted to improve your articulation. Having a large vocabulary can help, because using hyperspecific words can express more in certain scenarios, but you do not need to use big words to sound smart. You also shouldn't utilize words that you aren't confident you understand, because misusing words is a major detriment to people understanding you. Words don't always mean what their roots might imply they mean, though that's usually a good reference point. (As an example, I see people use the word "ambivalent" as a synonym to "impartial" when it doesn't mean that, and "enormity" doesn't mean "sheer size", it's exclusively a word to signal how immoral something is on a scale. For example, the quote "the enormity of my desire disgusts me" from Richard Siken does not refer to unmanageably sized desire, but instead, desire perceived as extremely evil or wicked. People think "enormity" is the state of "being enormous", an adjective that does refer to solely size, rather than a word of its own, because of how the word is structured. "Ravish" is often used as a synonym for the act of having passionate sex, when it super, really, extrremely doesn't mean that. Of course, people make mistakes.)
Honestly, I think a really significant part of it is just that I grew up autistic and I had to express myself a lot more concisely than other kids/teenagers, and it's a skill I gradually learned over time. It just translated to things like writing and posting about characters I really like on the internet.
TLDR: Read a lot and make sure you understand the words you're using.

Oh has it resurged? It's been around for a couple years anon. I haven't seen anything recently but my thoughts haven't changed since the last time I saw it. My thoughts are... hm. I mean, I think it makes more sense if abilities are... foreshadowing to trauma? Yosano kind of debunks the theory because Yosano was traumatized BECAUSE of her ability. Atsushi's ability forms in a system-like manner and his tiger is there to protect him, so it works really well with Atsushi, but the theory kind of falls apart when it comes to characters like Yosano and Chuuya, who weren't traumatized until their abilities were put to use/until Chuuya's singularity was created