In answering this question, it is important to consider authorial intent. In some ways these types of "form critical" objections are parallel to how Shakespeare's works are often disputed. For a defense of the authorship of Shakespeare's works, see David Kathman's site, Shakespeare Authorship.
Based upon the mostly undisputed letter of Paul to the Corinthian's, it appears that Paul's authorial intent in choosing styles of communication was to work at effective communication. For example, Paul writes in 1 Corinthians 9:
To the Jews I became like a Jew to gain the Jews. To those under the
law I became like one under the law...To the weak I became weak in
order to gain the weak. I have become all things to all people, so
that by all means I may save some.
In general, this philosophy might explain Paul's use of short, pointed sentences at times while, at other times, writing in sentences that are long and complex.
In his introduction to The Jewish War Josephus writes that he wrote two different versions of "the war of the Jews against the Romans" The first version he says was "in my vernacular," that is, in Aramaic, "for the up-country barbarians." While we don't have both versions, an intriguing possibility exists that there were variations in style between the two versions that was very substantial.
In the field of marketing, the use of "buzz words" to target selective social niches is very important. Paul, with a little help with his amanuensis (scribe), might have deliberately utilized textual outliers, such as "promise of life...with a pure conscience...from a pure heart...guard the deposit of faith," as forms of an idiolect or "buzz words" that Timothy could especially connect with. Paul calling himself an apostle, herald, and teacher would also be examples of utilizing different styles of vocabulary and rhetoric that is varied according to the recipient(s) of the communication process.