I've owned both the 2020 Trek Checkpoint ALR5 (105 equipped), and now a 2020 Trek Domane SLR (Ultegra Di2 equipped).

The Domane has more than enough clearance to be a proper "gravel" bike if that's what you want from it. But I do get where your LBS is coming from with the finish. I would never take my Domane off-roading. I paid too much for the Project One ICON paint scheme, and I just paid way too much in general for the bike. So if you're looking for a bike to use mostly on road, but sometimes off road...that's literally what the Domane is for. The Checkpoint is just a proper gravel bike, that can also be ridden on the road. My Checkpoint was gray, and would have hidden grime and dirt well. My Domane is "Molten Marble" which is a weird red mixture color...and the finish is WAY more smooth and glossy. This is mostly due to the difference between an ALR line bike and the SLR line of bikes...but still, it's something to consider. My Checkpoint was also "slow". You could just feel that the bike was working against you anytime you were on the road. The few times I took it on the gravel, it felt awesome, but that isn't where I ride, so there was no point in having a bike specially designed for that. The Domane, even with 32mm tires is just way faster, given the same rider and conditions. I have data that shows I was able to improve almost 15% with regards to climbing and straight-line cruising speed under the same amount of effort once I switched to the Domane. To me, that's pretty amazing!

My honest opinion is get the Domane which should come with 32mm and then grab a set of wider tires for when you want to go off-roading. I love my Domane and would never go back to a Checkpoint, but that's just my personal opinion.