I've shot with both the D850 / 14-24 F mount lens, and the Z9 / 14-24 Z mount lens in Iceland, and they both are top of the line camera/lens combinations. The Z 14-24 lens is sharper corner to corner, and this difference would probably be noticeable when shooting astro or northern lights photography. I dont think you would notice the difference as much (if at all) with your typical landscape shots (maybe on super low WA shots with a near foreground subject).
So for me the lens purchase determining factor for your trip would probably come down to how much astro you are planning to shoot. Otherwise, for mostly landscape photography, I don't think the difference in the lenses would be noticable enough to necessitate buying the Z lens (unless you just want an excuse to acquire the Z lens sooner, rather than later in your long term migration strategy)
As far as travel restrictions for getting gear to Iceland goes; earlier this year I took the following along with me to Iceland: Z9, Z8, Z f/2.8 trinity (e.g. 14-24, 24-70, 70-200), Z 20mm f/1.8, 1.4 TC, full sized Gitzo tripod and all the typical sundries (batteries, cards, etc.). I was happy to have all of that along with me for the trip, and I got some really nice aurora shots with the 20mm 1.8 (maybe that could be a lens consideration as well).
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