So let me get the basics out of the way here:
I'm 24, starting to race. I have a weirdly proportioned body. Long legs, long arms, short torso (82cm inseam, 177cm fingertip to fingertip, 62cm trunk, 169cm height, 120lbs/54.5kg ).
I'm currently still riding my size 54cm Tarmac seen here
(scroll halfway down to see current setup). With it's zero setback post and 70mm stem, the total reach from tip of saddle to bars in 50cm. I did get a fitting done a few months ago, but not much was changed from my current setup, but I did get a better understanding as to my personal geometry. Handling is weird on this bike, and I'm starting to think the zero setback post is overemphasizing some muscles because I've had some knee pain developing over the past few months.
Now with all that out of the way, I'm looking to replace the frame with something that will allow me to use a normal 100mm stem while still retaining the same kind of weight and feel as the Tarmac. The reach of the Tarmac is 387mm, so I need something in the 350-360mm range. The CompetitiveCyclist fit calc suggests I need something with a 50-51cm TT. Basically the only options for me here are a couple frames in the smallest sizes available, many of which are "endurance" style frames, and all of which will require me to use a very long seatpost.
I'm not really able to drop another $3-5k to get a custom frame at the moment, so I'm limited to what I can find used after I sell the Tarmac frame.
Some options I have looked at:
BMC GF01 in a size 48cm
Cervelo R3 48cm (very short headtube)
Cannondale Synapse 48cm
A problem I've encountered is that no one has these bikes in these sizes available to try out. Closest I was able to find for the Cervelo was a 51cm, and when I found a 48cm Synapse, the seatpost had to be beyond the minimum insertion and they wouldn't let me try it like that.
Any advice on the frames I've mentioned or those that I should consider?