I have been to Iceland and with the weather you are likely to encounter (particularly wind), I would bring the heaviest drone you have. I had my
M2P with me and there were many times I simply couldn't fly - if I had a Mini with me I would have flown even less.
Also, most of the places you will want to fly (i.e. every tourist stop, major waterfall, major landmark, etc.), all drones are prohibited anyway (look for the signage), so anywhere you do fly you will likely be the only person for miles, especially if you leave the area around Reykjavik.
Regarding your
Mini 3, some lens decentering is normal on all lenses, even full frame camera lenses costing thousands. It's also much more prevalent on wide angle lenses of all types, unfortunately. You need to do some controlled testing, and if one side or area of the image is extremely soft compared to the opposite area of the image, and that is repeatable, then you probably have a decentered lens. If you're looking at the extreme edges and corners and finding they are quite soft, that's normal.
Another thing to keep in mind is that at hundreds of feet in the air, fine details have almost no resolution on them, so when you zoom in, it's going to look soft due to the low pixel density. Other things can also affect sharpness such as atmospheric distortion and heat distortion, but that will affect the whole image. You probably already know most of that, but just in case.
Another thing to consider is swapping to a nocturnal schedule when you're there to avoid crowds, since you will have nearly 24 hours of daylight. I frequently went out around 1-2AM and it was nice and bright with absolutely zero people.