Personally, I’d go for a new bike. CF frames have improved dramatically with frame builders learning to dial in ride quality, designing stiffness where you want it, and compliance where you want that. All at a lighter weight.
For example, I thought my 2005 Giant TCR team, a top of the line bike at the time rode great. However my 2013 Willier Zero 7, surpasses it in every dimension. It handles more confidently because the front end is stiffer; the Bottom bracket flexes less, and yet the ride is more comfortable. By comparison the Giant rides like a brick.
Your Trek is a generation behind my Giant. I test rode your bike before buying my current Merlin. At that time CF builders were focused on stiffness, and strength. That lead to a ride that imho, feels both dead, and comparatively uncomfortable.
Current CF bikes, 7 years newer than mine have continued to improve. And there are numerous options to get exactly the qualities you want in a ride.
If it’s in your budget, I would definitely go new rather than spend a lot on a very early CF bike, which is now about 4 generations behind in the development of CF frames.
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