Learn English – Why are “nation” and “national” pronounced differently

It's not trisyllabic laxing in the strict sense, since the stressed vowel is in the second-to-last syllable rather than the third-to-last in normal pronunciation. However, some researchers seem to consider all laxing processes in English to be related at an underlying level. I've just read a paper, "English Syllable Structure and Vowel Shortening" by Balogné Bérces Katalin, which talks about -ion shortening (seen in words like divide, division); it mentions that this only laxes the vowel i (for example, invade, invasion does not show laxing). This seems to be the relevant process here, although the suffix is not -ion but -ient. In a parallel fashion, the only vowel affected is i; words like quotient and patient have tense vowels. (This is one way this process differs from normal trisyllabic laxing, which applies to all of "a," "e," "i" and "o" as in insane/insanity, serene/serenity, divine/divinity and verbose/verbosity.) I'm still working on processing the paper and understanding the reason why this process applies, but Balogné Bérces connects it to the y-sound in the suffix. This palatal sound is also clearly the cause of the /ʃ/ ("sh") sound in sufficient, compared to the /s/ ("ss") sound in suffice. Neither of them is normally pronounced with /z/, so I'm not sure why it sounds like that to you.

I believe the difference between "office" and "suffice" is because the former is a noun and the latter is a verb; disyllabic nouns in English tend to be stressed on the first syllable, and disyllabic verbs to be stressed on the last.

Consonants, as Ladefoged has said, are just different ways of starting and ending vowels. The difference you are hearing are the two different ways of ending the vowel. Bringing the tongue dorsum up to make a complete closure with the velum is a relatively slow gesture that changes the resonant properties of your mouth.

Raising the tongue dorsum up towards the velum causes the second and third formants to move towards each other (phoneticians call it the "velar pinch"), and because the gesture is slow, you can easily perceive the change in the way the vowel sounds during the time it takes for the closure to be made (the pure vowel should have steady formant if pronounced carefully). The gesture for making /t/ is done with the tongue tip, and it is faster and produces less of a disturbance on the vowel. It produces less of a disturbance because vowels are made with the tongue body, which can stay still while the tongue tip flicks up.

However, many English speakers draw their vocal cords together as they form a word-final /t/, creating a creak in the voice or even a complete stop in airflow (unrelated to the stoppage caused by the tongue tip gesture).

Make a sound spectrograph using praat and you will see more vividly the differences you are hearing. The waveform itself is normally not useful except for illustrating sound intensity.