Honestly you are gonna love the R5ii! It is such an amazing beast for landscapes and I am so excited for your Iceland trip. I have spent a ton of time hiking with both of these lenses and for a trip where you are constantly on the move, the Canon RF 24-105mm f/4 L IS USM is the absolute winner imo. Here is why I think it fits your $2500 budget and the R5ii requirements perfectly:

• Versatility: That extra reach to 105mm is massive when you are trying to frame a distant waterfall or mountain peak. Youll really miss that extra 35mm if you only have the 70mm.
• Sharpness: People get way too caught up in lab charts. I use this lens on high megapixel bodies and the files look crisp and professional. Unless you are printing massive gallery walls you wont notice a difference.
• Weight: It is significantly lighter which matters a lot when you are trekking through Icelandic wind and rugged terrain for hours.
• Cost: It leaves you with over $1000 leftover from your budget. You could use that to grab a Canon RF 35mm f/1.8 Macro IS STM specifically for low light or northern lights shots! The Canon RF 24-70mm f/2.8 L IS USM is fantastic glass but it is heavy and honestly kinda overkill for landscape work where you are usually stopped down to f8 anyway. The weather sealing on the f4 is top notch so you dont have to worry about the mist and spray. Go with the 24-105mm and enjoy the extra reach!