You think that, because it's what you want to believe, so it becomes the only way you can see it.
But, as mentioned in an earlier post, I have personally heard (probably on more than one occasion) vpw plainly and very clearly say (to paraphrase) that when it was written, there was no editing of it. None whatsoever. Paul didn't start to write it one way, then back up, change a word, then continue writing. It flowed out, and that was it.
Which is dang near the opposite of what you are supposing. Personally, I think it makes no difference how long before any bits and pieces (or chunks) of truth were revealed and made known to Paul. I suspect most (if not all) of what is later referred to as "his gospel" was revealed to him over a course of time spent in Arabia after his escape from Damascus, years before the school of Tyrannus, which also was years before what was written to the Thessalonians. And to put it bluntly, VPW said a lot of things in WC teachings that were just his opinion, and should be as viewed nothing more... or you'll miss things (concerning the truth) a lifetime, and never know it.
For instance, Paul taught at the school of Tyrannus (prior to his trip to Thessalonians.) He also taught in Thessalonica (duh...)
Why would anyone think that much of what is written in Thessalonians was "brand new" to them, or that they had never heard any of it before?
So, why "limit" the truths in it to just having been taught to Timothy and Silas prior to it being written? Makes no sense.
Neither do I think that Paul was diligently practicing, rehearsing, or in any other way deliberately "getting ready" to write Thessalonians ahead of time. Was God preparing him ahead of time? Of course. That doesn't say or mean that Paul knew what (all) God was prepping him for. After all, who ever does? But when it was time... write! And, there it was.