I fail to see how its only 4cm difference between my current and the Dale.
4cm is significant enough for the likes of Cannondale to invest in moulds to produce 3 frame sizes within it (eg. 54, 56, 58).
On paper it looks ok sizewise but the standover is that close? Does it ‘really’ matter? On a MTB yes but a road bike?
I’m a big fan of ‘Dale road bikes but they are “old fashioned” in having horizontal non compact top tubes, so look and feel big for their size. You might not ‘need’ stand-over as you’re not a 9 year old doing your cycling proficiency, but it may psychologically as much as physically feel too big.
As well as the head angle have you compared head tube length? the ‘Six is their racey frame and has a shorter head tube than the Synapse and no doubt the P-X. How many headset spacers are you going to need to replicate your current riding position.
If you can’t or wont sit on a 58, get the full geometry written down and transpose this onto your current bike, with string and card if need be, to give you an idea how it would look and feel. I’m 5’10 and a 32″ inseam, some size guides put me on either a 54 or 56 C’dale, but having done this exercise on my current bike I convinced myself the 56 would feel way too big for me and saved myself the £2800 I was toying with spending on a discounted Hi Mod Dura Ace.
Devils Advocate but you’re new to road riding, I returned to road a few years ago and my second bike (bought 18 months and may miles after my first) is longer and lower than my first as I have adapted; becoming more flexible and stronger in the core.
I’d hold off upgrading until you have more miles on the P-X, are more comfortable and confident as roadie and have identified what you consider shortcomings in the P-X.
