The Soma’s heavier weight and relative flexiness compared to your carbon bike is what it is. But you’ll appreciate the longer chainstay length, the wider tires, and the ability to carry stuff.
Try lowering your gearing if you haven’t already. Being able to spin lower gears is something I appreciate on long rides. I use 48/32 and 44/28 with 11-32 cassettes on my road bikes.
Use the widest tire you can on the Soma, on the widest rim.
Think of them as two different bikes for different purposes, and not as your fast vs slow, or light vs heavy bikes.