Thanks for dropping by my SparkPage and leaving a comment about your fatbike. If memory serves, you have a Motobecane Boris X7. I am glad you are doing a lot of good winter riding.
You mentioned getting a Fatback Corvus for all-season riding. Are you considering 4" to 4.2" wide fat tires? Or are you looking more for 29+ with 3" wide tires?
The 29+ are high-traction trail bikes. Maybe these bikes would do okay on snow.
Your Boris is very heavy at 40+ pounds. About 18 pounds of the bike's weight is the wheels and tires. Heavy wheels and tires are a double penalty. The wheels and tires add to the overall weight but also increase rotational mass. As you pedal the bike the heavy wheels and tires resist rotation. This phenomenon is called a high polar moment of inertia. Heavy rotational mass also tends to resist steering input which can make a bike feel unresponsive.
The main advantage to a carbon frame is not weight savings but stiffness. Carbon bikes tend to be more responsive. The actual frame weight savings of carbon versus high-quality aluminum may be 1 to 3 pounds. Carbon frames are found on higher-end bikes. High-end bikes that have lighter components such as light handlebars, seats, seat posts, wheels, tires and drivetrain.
My 9:Zero:7 fatbike has a 7005 heat-treated aluminum alloy frame that is triple-butted and very feather light.
The wheels are Surly Holy Roller Daryl with Hope Brother Hubs and DT Swiss Alpine III spokes. The wheels are fitted with 120 TPI Surly Nate tires that have been converted to tubeless. The tubeless conversion saves 3 pounds per tire. Combined weight of both front and rear tires are under 10 pounds.
My 9:Zero:7 is fitted with a Race Face Turbine 22/38T crankset, Shimano XT 11x36T 10-speed cassette. These items are very light and carefully machined. The Race Face Turbine crank costs $300. The XT Cassette goes for $85. The handlebar, stem and seat post are Race Face Turbine level that cost $70, $94 and $80 respectively. The total cost for these few bits. Add in a KMC X10SL Chain for $80 and Specialized Titanium Phenom Seat for $130 and you are pushing $840. Just for a some really light components, I paid $140 more than you paid for your whole bike.
My medium 9:Zero:7 weighs under 30 pounds, which is light for a normal trail bike and feather weight for an fatbike. My custom-made 9:Zero:7 cost $3,600. The bike was built by Golden Bike Shop in Golden, Colorado to my exact specification. Brakes, shifters and derailleurs are Shimano XT.
This bike rips on dry trails or deep snow. Due to an aggressive geometry, the bike is unbelievably responsive. I can churn through moderately deep snow at 30 mph. On rocky or rough dry trails the bike is a bouncy but fast.
I have great trail bikes so I don't use the 9:Zero:7 much in the summer. My fatbike is in it's element when there is snow on the ground.