I've been riding an SL6 Domane for 3 years, about 95% on road - the occasional gravel stretch of a few miles during a 50 mile road ride, and the occasional crushed limestone rail trail. I run 32mm tires.
The Doimane is not great on actual gravel and the tires I run (Continental GP 5000s) are probably the biggest downside. The Domane and those tires are OK for the riding I describe above but I wouldn't do a "real" gravel ride on those tires, and since I weigh 230 I'm not convinced the rear wheel (Vision 40) and spokes are good choices for gravel either. But, if your non-paved riding will be mostly crushed limestone rail trails, I think the Domane will be fine. On that type of riding on the Domane, I haven't seen any finish damage - just lots of dust!
Since I want to do more/longer gravel rides, I considered buying a second set of wheels for the Domane. That would be a good compromise.
But, I have an old beater hybrid bike that I had been using on pure gravel or rail trail rides, and a old Trek 520 that I use on multiday road tours. In the end, I decided to do a "N-2+!" - I donated the old bikes and bought a Jamis Renegade steel bike as a touring/gravel bike.
I could have saved money by buying the wheels, but this way I could justify a new bike by ending up with fewer bikes! But, it is a cheaper alternative. If you aren't a "clydesdale" like me, you could just swap tires for actual gravel rides but to me that is a pain and increases the chances of nipping a tube (I have not gone tubeless) and causing self induced flats.
Armor for carbon finish: on my 2017 Domane, the rear brake and derailleur cables are external from the levers to the down tube, then they are routed internal to the downtube. Where they go around the headset, the cables chafed a bit. So, I put clear contact paper/shelf liner over the wear spot, which is what I've done for years on bikes and kayaks at wear points. Similar to what most bikes have on the chain stay under the chain. You could put some of that on the bottom of the down tube if you were worried about the finish.
I'm really enjoying the Jamis and the 1x drivetrain, but I really, really love that Domane and those Vision 40 wheels. If my riding mix went to 50/50 and I had to choose one bike only, if I could afford spare wheels I would definitely choose the Domane. If I couldn't afford spare wheels, and the 50% gravel was all rail trails, I'd probably stick with the Domane and live with higher rolling resistance tires on my road rides (I'm not a racer, but do like keeping up with the group on group rides!) that have some tread and better puncture resistance.
If that 50% was going to be more dirt roads and loose gravel, I'd probably go with the Checkpoint. I went with the Jamis steel bike because friends who have them are fanatics about Jamis, steel was cheaper than carbon.