It's a very good week when I can spare enough time to crest 100mi - 300mi is huge!
I presume you've changed tires already? Switching to nice kevlar (folding) bead tires is an easy way to save 1/2 lbs over the OEM wire bead tires.
Beyond that, I would look towards switching wheels out. Given that you're on a thru-axle, disc brake bike, an upgrade to carbon wheels could not only save you at least 1 lbs (if not closer to 1.5lbs), but also be more aero, and have the added benefit of being compatible with almost any new frame/bike. Something like Hunt Carbon Aero Disc wheels will be a huge upgrade over your current wheels, and are likely better (or at least as good) as wheels that come stock on bikes costing up to about $4-5k.
You can switch out your bars the next time your bar tape starts wearing out, which should be fairly frequently if you're riding 300mi week. In the meantime, a switch to a carbon seatpost from the OEM alloy post could net you 1/3-1/2 lbs, and only cost you about $100-150. I have a U.S.E. Evo on one bike that comes in around 120g, which is better than my Thomson Masterpiece (about 160g IIRC), and both are definitely better than the OEM FSA that I've retired (300g+).
I wouldn't bother upgrading the Sora to 105 - not because 105 isn't better, but because you're going to have to change so much going from 9-speed Sora to 11-speed 105. At a bare minimum, you're looking at a new right shifter, chain, cassette, and rear derailleur, and for optimal performance, you're going to want to change out the left shift, front derailleur, and crankset, as well. At that point, you're probably looking at $600 or more in parts, plus labor if you're not doing this yourself - not to mention rewrapping bar tape (an expense I've forgotten to factor in before). All in, you're probably looking at $1k to upgrade to 105 - and that's sticking with mechanical brakes. Add a few hundred on top of that if you want to switch to hydro.
My recommendation would be to upgrade the wheels, tires, and cockpit of your Domane, and keep riding the hell out of it. While you're doing that, figure out the things you like and dislike the most about the Domane, and narrow down your choices for your next bike - and you'll be able to port your nice wheels and (likely) your cockpit to your next bike.