In greeting the Thessalonians, Paul is following the standard template of thanking his audience for either past kindness when he visited them or some kindness that was reported to him.
We give thanks to God always concerning all of you, making mention
constantly in our prayers, because we remember your work of
faith and labor of love and steadfastness of hope in our
Lord Jesus Christ in the presence of our God and Father,
Notice the "faith, hope, love" trinity. Here "work" (ergon), "labor" (koros) are synonyms, used so as not to sound repetitive. The semantic ranges are not identical, ergon suggests work done out of a moral obligation so "duty" is sometimes used and "koros" suggests work done for a wage, but they are both types of work and are interchangeable here.
The premise of the question -- that there is only one possible ergon in all the universe because Paul thanked the Thessalonians for their ergon of faith when he last visited them - is an unsound interpretation. Fantastically unsound.
If A tells B, "thanks for your gift of fish", and then A tells C "Thanks for your gift of eggs", then we do not need to be all confused, saying "I thought there was only one possible gift that could be given, the gift of fish, so what is this egg? How can I interpret this cryptic passage?"
Proof texts are not needed, but just for fun we can point out that Paul referred to "living in the flesh" as fruitful work (as opposed to dying) in Phil 1.22. And in general, this verb was used to describe both good and bad deeds of many different sorts and as a generic "deed" as in James 1.22 (be a doer who does works (ergon) not a hearer"). In 1 Tim 3.1, being a supervisor is referred to as a good work. "The saying is trustworthy: if anyone aspires to supervision, he desires a good work. (ergon)", etc.