Learn Russian: Текст не то чтобы посредственный, а немного затянутый. - The text is not exactly mediocre, but a little drawn out.

The phrase не то чтобы softens the statement. While не [A], а [B] is a hard correction meaning 'not [A] at all, but rather [B]', adding то чтобы acts as a mitigator. It translates closer to 'not exactly [A]' or 'it is not that it is [A]', making the criticism of the text sound more polite and nuanced.

Why do we use the conjunction а here instead of но for 'but'?

The construction не то чтобы ..., а ... specifically relies on а because it is a corrective structure. You are rejecting the first idea (посредственный) to replace it with the second (затянутый). The conjunction но is used when both facts are true and coexist, which would not work with this 'not X, but Y' replacement pattern.

затянутый translates to 'drawn out', but what kind of word is it structurally?

It is a past passive participle formed from the perfective verb затянуть (to drag out, to tighten). Like many Russian participles, it functions entirely as an adjective here, taking the masculine nominative ending to match the subject Текст.

Could we use the short-form adjectives (посредственен, затянут) in this sentence instead of the long forms?