Three things define the fit. The TT length, the STA and head tube length. The TT length and STA work together to define the reach. It's rare when the HTA has enough of an effect to worry about.

If you measured your inseam to saddle like crotch contact in bare feet, I can't imagine how you're riding a frame with a standover that's 5cm greater than your inseam. What's your actual saddle height? Mine's 73cm, for example.

I'm only a half inch taller, but my inseam is 83cm, so standover is never an issue. Too much reach is a greater concern for me.

The big difference with the Synapse is the much taller head tube. If you don't want much saddle to bar drop, that may be OK, but the head tubes are too long to suit me.

I'd compare the 50cm CAAD9 and the 51cm Synapse. The CAAD9 would have only about 4mm more reach and a 30mm shorter head tube.