Paul taught the word of God-the doctrines of the Lord as they were delivered to him by revelation. Thus, the 'traditions' that he would encourage them to follow must be those doctrines of the Lord. Why he chose the word 'tradition' over 'doctrine' in this case is perhaps a good question to pose, but I suspect Paul would not demand they hold to traditions that were anything other than the doctrines of the Lord.
And excuse me if I used the term 'true doctrines' trying to clarify them away from what people might normally consider a 'tradition' to be, ie, a tradtion of men. The Bible mentions doctrines of God and doctrines of devils-it shouldn't take much discernment, particularly considering the context of this discussion, what I meant when I used such a term.
My reasoning in not including the first definition of 'tradition' here is that it doesn't fit the context. ie, Paul delivered traditions to them-so he traditioned the traditions to them? That one clearly didn't fit the context of the meaning in the verses you gave, while the second two would. That I included the definiton numbers of 2 and 3, AND mentioned that I was only including the ones that fit the context, wasn't at all disingenuous-to remove them and pretend the first didn't exist would be disingenuous. Just didn't see the point in including it-waste of space.
As for the 'traditions', or the practices delivered by Paul to them, it would include such as:
"Furthermore then we beseech you, brethren, and exhort you by the Lord Jesus, that as ye have received of us how ye ought to walk and to please God, so ye would abound more and more.
For ye know what commandments we gave you by the Lord Jesus.
For this is the will of God, even your sanctification, that ye should abstain from fornication:
That every one of you should know how to possess his vessel in sanctification and honour;
Not in the lust of concupiscence, even as the Gentiles which know not God:
That no man go beyond and defraud his brother in any matter: because that the Lord is the avenger of all such, as we also have forewarned you and testified"
And other traditions or doctrines, or mentioned here, commandments, that Paul taught the Thessalonian church in the epistles to them, as well as anything they learned from Him in other epistles that had been passed to them from other churches, or taught to them by word of mouth when He taught them in person.
No reason to only concern ourselves with just the second epistle-I would believe all his teachings would be under consideration. Notice paul says in 2:15, 'whether by word or our epistle" he sent them two epistles, and he also includes word, hence, oral teaching.