a really interesting question for chiaki !!

for a short answer, chiaki's speech quirk wasn't that different from the way he currently speaks (i.e. 1st/2nd year chiaki uses ~dazo, ~da, ~da na, ore for first person, the rough-sounding suman for sorry, etc just like he currently does). but his demeanor and the way he speaks to others definitely went through quite a few changes as he grew older. he started out speaking a bit more politely until he started to make more close connections in his 2nd year, and then going for a complete "character change" in his 3rd year (expressing a more confident version of himself at all times)

long answer: the general way chiaki speaks in japanese is shared between him and characters like adonis, hokuto, keito, etc. it's sort of the classic "masculine-sounding informal speech", if a female character spoke like them she'd also give off a masculine feel. this form of speech can also make a character sound more "proper" (for example, this sort of character wouldn't be the type to use contractions like goin', somethin'). and one thing to note is that it isn't necessarily "polite", and can quickly make the person sound rude depending on their word choices/tone. for ex. hokuto never switches to a politer way of speaking when talking to his superiors, and the way he naturally speaks makes him sound condescending, so he ends up sounding rude to them.

these characters do have a few differences from each other, usually found in their word choices and tone. for example:

• keito uses much more rigid language sometimes (using a more archaic way to say "no" sometimes, for ex.)
• whereas adonis can sound a bit simplistic japanese (short/basic sentences)
• and hokuto can sound dry in his responses sometimes
• meanwhile, chiaki sounds a bit more energetic compared to them (he uses ~dazo way more than keito for example, and that can give off that more bold energy)

it's hard to put it in words because it's like, both Vibes but also japanese linguistic shit 😭 the way i described it above is mostly how i personally differentiate his tone from characters who speak like him. my main point is, chiaki doesn't have a "unique" speech quirk like shinobu (degozaru/sessha) or kanata (full hiragana), but his speech quirk definitely gives off a certain personality (speaks in a proper tone, with a lot of energy, can sound bold, etc)

note: actual, real-life japanese speech is usually different from how it's used in fiction. a lot of japanese fictional characters are characterized "unrealistically" to better convey their personality to readers/viewers. for example japanese people don't use the second-person pronoun "you" very often in speech (they're more likely to refer to you by name), but those various "you"s japanese has is used in fictional stories to convey the tone the character is giving. i'll be talking about his changes based on that

so! in terms of chiaki in his 1st, 2nd, 3rd year, and then post-graduation, most of the differences is mostly in his word choices/behavior when with others, and the way he presents himself to other people. OK! that's probably enough context... now! time for said notable differences!!

NO.1

chiaki uses kimi for "you" when talking to people, whereas current chiaki uses omae. kimi gives a friendly and polite feeling. if someone says this in an anime to a stranger, it feels "neutral" polite, not rude and not overly polite either. omae on the other hand sounds much more informal, and can be used to show superiority (for example, a villain might call the hero "omae" to indicate how much they look down on the hero, or a boss will call his subordinate "omae")

this becomes a plot point between chiaki and subaru in his 2nd year chapter 3, where chiaki goes from kimi to omae with subaru. at first subaru's put off by the switch because it's like "who tf do you think you are calling me by that..." (since it sounds more casual and therefore feels rude) but chiaki explains himself that using kimi might've felt like they were strangers to each other so that's why he switched to something more casual. the logic being that, since omae is such an informal second-person pronoun, it can indicate closeness if you use it with someone you feel close to, unlike kimi that's a lot more neutral and doesn't really truly indicate anything special about their relationship. (if... that makes sense. i think my tl note for that scene makes it a bit clearer)

another person he switches from kimi to omae with is kuro. in their 1st year he'd say kimi, but in their 2nd he switches to omae, indicating their closeness as friends.

other than those two, chiaki sticks to kimi for everyone else in his 1st and 2nd year, very fitting for how he felt pretty distant from kanata and madara until he finally got to know them better. in his 3rd year, he completely switches to using omae with everyone, and if he uses kimi it's to either sound dramatic or talking to a stranger (or anything like those situations).

NO.2

this one's obvious but chiaki uses -kun for everyone, which again, gives a polite and friendly feeling.

he stops using -kun with kuro first by their 2nd year, indicating their closeness. but -kun doesn't necessarily mean distance, just that not using -kun sounds more close like "woah those two know each other" kind of feeling.

ONE SPECIAL NOTE THOUGH that there's this moment in chiaki's 1st year where, when madara is mentioned, he uses "mikejima-kun" for the first few times, but then eventually within the same conversation, goes with "mikejima-san" and sticks to that for the rest of it. i'm still not sure why it took him a few mentions to switch entirely...?? but there's that. he almost immediately switches to showing respect to madara from the 1st year.

NO.3

this one's the easiest to convey in the translation, since it doesn't have to do with how japanese speech quirks work. it's much more common in his 1st year, but chiaki speaks a lot more unsure or "plain" to people in terms of his word choice and tone. for example, his speech quirk is still the usual informal rough speech, but he asks something in a courteous manner.

in other words his personality itself was more "passive" in the past, not really treading on people's boundaries and stepping back when he's told to. or asking questions in an.. innocent? manner, and so on and so forth. fitting for his naive 15-16 year old self at the time.

he picks up much MUCH bolder ways to speak to people in his 3rd year, since in his 3rd year he adopts the "hot-blooded passion" demeanor. once he graduates he mellows down again but not 100% to the same level as his 1st/2nd year. just like... mellowed down and chill now because he's not a teenager living the best days of his high school level anymore LOL

aand i think those points are the most important ones...?

notable things that chiaki has said in general whether in his 1st year, 3rd year, or right now:

NO.1
there are a few moments here and there where chiaki words himself in a more meek or unsure manner and starts speaking more politely (when he gets scared that there's a ghost in 1st year chapter 1, when kuro first appears and freaks him out in chapter 5 ). he still does this sort of thing nowadays too when he gets scared, happens as recent as vs gourmet, praise of delicious flavors 9 (cute).

sweet halloween is the one that did this a LITTLE differently where he kinda talks more casually and plainly to express politeness. it's mainly in let's party 2, the "takamine-kun" line. i think, because he's talking to midori (someone younger than him), he shows that politeness by AVOIDING talking in a rough/informal manner, rather than showing it through wording himself in an OVERLY polite manner (which is how you would speak to someone older than or above you, and that doesn't work here when midori is younger than him)... (there's a lot more implications in that switched speech tone but that is an Entirely Different Topic)

NO.2
chiaki typically uses suman or sumanai to say sorry, which fits with his usual informal rough speech. but every once in a while, especially when he's showing a more vulnerable or weak-hearted side to himself, he says gomen, which is much softer in comparison to his usual way. this is shown in various stories, from meteor impact (switching between suman and gomen depending on the "emotion" he's feeling at that moment), to sweet halloween, comet show and even stella maris.

i think there's a few more things i could discuss about these points, but i can't fully remember them atm and the answer is long enough as is HAHA

all in all you're absolutely right to pick up the softer, politer tone, and it is indicated in some word choices in japanese!!! thank you for asking such a fun question hehe he's so cute... and sorry for taking a while to respond, i wanted to give this question proper attention because i find this part of japanese AND chiaki so much fun so...!! let me know if anything was unclear or if you have other questions!