I find the Trek FX Sport 4 to be comfortable but much less fun to ride than the Specialized Sirrus 4! Unfortunately, on the Sirrus I feel like I'm arching my back and want to reach further forward. Moving the seat back helps, but it doesn't resolve the issue. The Sirrus has a 32mm higher stack and 14mm shorter reach, but otherwise has a nearly identical geometry (link to full geometry comparison below) Can a professional bike fit compensate for these 10-30mm differences and make the Sirrus as comfortable as the FX Sport? I assume that adding a stem several cm longer can address the problem?
For context, I am looking for a flat bar road bike for use on group rides on paved roads. I have chronic lower back pain that prevents me from using drop bar bikes. I'm honestly not sure why the Sirrus feels so much more fun, but it just feels like I'm more "connected" to the Sirrus. The FX Sport ride quality feels... Dull? Maybe it's because the Sirrus has narrower tires vs the FX's gravel-like tires? Regardless, I would like to know your thoughts on the geometry question.
P. S. I have tried smaller sizes, they do feel less comfortable for both the FX and Sirrus.
Update: I spoke with Specialized support, and they said that I can get a bike shop to swap a part in the suspension stem system to reduce the stack by 15mm. Specifically, the bike comes with a 15mm rise headset cap for the future-shock stem, and it can be replaced with a 0mm rise headset cap.