Hi pcclay,
There's already lots of advice on trying to sort out your position and comfort on your current bike.
However if you try that and it doesn't work have you considered any of the Kona or Fairlight bikes?
I have a Genesis Day One 2014 model and although I've persisted with it to get sort of comfortable on it unfortaunately I can't get it just right for me because of the geometry. I've tried to find the reach and stack measurements for my frame but try as I might I can't find them online now and even the Genesis catalogue doesn't list them.
http://www.genesisbikes.co.uk/img/uploads/files/genesis_cat2014_web.pdf
It did however state that for my 56cm frame is the headtube length is 150cm. Anyway with this bike, which also has a seat tube angle of 74 degrees, I've never been able to get really comfortable for long distance rides because I'm too stretched out and there's a drop to the bars (which I have worked to minimise as much as possible). I do find some of the Genesis geometry decisions a bit interesting and contrary to other manufacturers. Take for example the seat tune angle which for my 2014 model was the same 74 degrees for all of the frames from 52 to 60 cm. You can find the stack and reach for the 2015 model and it must have changed a bit because they slackended the aggressive 74 degree seat angle ever so slightly and the 2015 geometry for the Day One was:-
2015 Genesis Day One Alfine 2015
Reach: 383 mm (or for the 58cm frame it was 391 mm)
Stack: 596 mm (or for the 58cm frame it was 619 mm)
Head Tube: 175 mm (or for the 58cm frame it was 195 mm)
ST Angle: 73 (or for the 58cm frame it was 73)
So for the purpose of this comparison the Genesis Day One bike is at least similar in geometry to your CX bike.
Anyway since having the Genesis I subsequently got a 2016 Kona Roadhouse which has for my 56cm frame:-
Reach: 388 mm (or for the 58cm frame it was 396 mm)
Stack: 578 mm (or for the 58cm frame it was 593 mm)
Head Tube: 145 mm (or for the 58cm frame it was 160 mm)
ST Angle: 73.5 (or for the 58cm frame it was 73)
So again for comparison to your bikes the Kona Roadhouse (apart from a shorter HT length) is much more similar to your Cannodale Synapse.
For me personally I've been much more comfortable on this Kona than I ever have been on the Genesis. Admitadly it's a completely subjective thing and horses for courses and so for example some people who race might not like my Kona because they want a more aggressive, more strteched out, lower and aero position than I do.
This is the Roadhouse that I got and I was really lucky because I got this 2016 model at a big discount when the 2017 one (which had a big price increase) had been released:-
http://road.cc/content/review/182237-kona-roadhouse
View: https://youtu.be/ltLN1bOXS10
Therefore you could look at something from either the 2017, if you can find a reduced bargain, (
http://2017.konaworld.com/bikes.cfm ) or 2018 (
http://www.konaworld.com/bikes.cfm ) Kona range.
2018 Kona Wheelhouse 58 cm
Reach: 391 mm
Stack: 620 mm
Head Tube: 195 mm
ST Angle: 72.5
2017 Kona Wheelhouse 58 cm
Reach: 391 mm
Stack: 619 mm
Head Tube: 195 mm
ST Angle: 72.5
2018 Kona Rove NRB 58 cm
Reach: 400 mm
Stack: 630 mm
Head Tube: 207.7 mm
ST Angle: 72.5
2018 Kona Rove ST 58 cm
Reach: 400 mm
Stack: 630 mm
Head Tube: 189 mm
ST Angle: 72.5
2017 Kona Rove ST 57 cm
Reach: 392 mm (or for the 59cm frame it's 398 mm)
Stack: 603 mm (or for the 58cm frame it was 622 mm)
Head Tube: 160 mm (or for the 58cm frame it was 180 mm)
ST Angle: 73 (or for the 58cm frame it was 72.5)
2018 Kona Sutra Ltd 58 cm
Reach: 395 mm
Stack: 640 mm
Head Tube: 191 mm
ST Angle: 72
2017 Kona Sutra Ltd 58 cm
Reach: 395 mm
Stack: 640 mm
Head Tube: 191 mm
ST Angle: 72
For another option have you looked at British company Fairlight Cycles?
They do proportionaly geometry which you can learn about on their website but essentially to accomodate different body shapes and riding charactristics/ geometry preferences they do a Regular and then also a Tall version of each frame size:-
https://fairlightcycles.com/why-fairlight/fit-function-form/?v=79cba1185463
Proportional Geometry Video:-
https://fairlightcycles.com/?v=79cba1185463
Fairlight do a rather georgeous looking all season all road bike called the Strael and this was the bike on which last years TCR winner James Hayden rode (
http://road.cc/content/tech-news/22...ce-2017-winner-james-haydens-fairlight-strael ).
Strael 58cm Regular
Reach: 402 mm
Stack: 583 mm
Head Tube: 160 mm
ST Angle: 73
Strael 58cm Tall
Reach: 389 mm
Stack: 619 mm
Head Tube: 200 mm
ST Angle: 73.5
Looking at the above the Strael 58 cm Tall is pretty similar to your Cannondale Synapse figures.
However if you're after more of a bike that has greater flexibiluty (tyre size etc.) then you could look at their Faran adventure bike the Faran:-
Faran 58cm Regular
Reach: 400 mm
Stack: 587 mm
Head Tube: 151 mm
ST Angle: 73
Faran 58cm Tall
Reach: 385 mm
Stack: 624 mm
Head Tube: 191 mm
ST Angle: 73
Enjoy your search for a new bike and good luck finding the right one for you!
John