I'm headed to Iceland in a few weeks and have been going back and forth on dealing with bringing the DSLR but I think this seals the deal. I think we'd all consider 4x6 on the small side of a print and if an iPhone doesn't provide decent prints at that size it has no hope at 8x12 or larger.
You might want to consider photographer insurance or a third-party warranty with weather damage coverage that covers you if it happens outside the country you're insured in. Camera companies boast about their weather sealing, but they don't guarantee it. You can get camera parkas to reduce the risk, but it's still a risk.
To be clear the iPhone shots are still great, but the DSLR is a cut above for certain scenes, e.g. low light portraiture. (full frame, 24-70mm f/2.8)
Sticking with one high quality lens & bringing no accessories helps a lot with the overhead. Pick up a tiny USB battery cradle & reuse your phone charger...
For low light portraiture get yourself an aps-c mirrorless with a modern 56 or 30/1.4 lens and get both image quality and portability, no need for full-frame now outside of pro sports or wildlife.
