Like someone mentioned, f2.8 is king for stars, but in my experience the Canon RF 14-35mm f4 L IS USM is way more practical for hiking. Grab a Breakthrough Photography X4 CPL with the savings.

Regarding what #1 said about "Honestly heading to the Southwest with an R5..." - weight is a safety issue. Hiking there for years, I once slipped because I was too top-heavy. Are you shooting the Milky Way? If so, you need the Canon RF 15-35mm f2.8 L IS USM. The Canon RF 14-35mm f4 L IS USM uses software correction that smears corner detail. TL;DR: f2.8 for astro, f4 for safety.

Regarding what #3 said about the f4 being way more practical for hiking, i definitely agree. Weight is basically a reliability factor because if youre gassed at the end of a 10-mile hike, youre way more likely to fumble your gear or just get sloppy. I think I heard some people mention the f4 lens might actually have better flare resistance when the sun is hitting it directly, which is huge for those desert vistas. Not 100% sure tho... IIRC it had something to do with the element coatings. Honestly, the Southwest grit is the real gear killer. Someone told me the weather sealing on both is top-tier, but id still be careful during a sandstorm. If you go with the lighter glass, you can carry a more stable tripod which is probably better for your landscape shots anyway. f2.8 is nice but hiking with a brick all day sucks. Just my take...

Honestly heading to the Southwest with an R5 is a total dream setup but you really gotta be careful about the weight factor when youre out on those long trails. I did Zion and Bryce last year and man, my back was killing me after the first day of hiking. If youre doing those long treks, the lighter f4 glass is usually the move. You might want to consider how much you actually shoot at night tho... if the astro stuff is just a maybe, carrying that heavy f2.8 brick all day is just a lot of extra work for a few shots. I would suggest just sticking with any of the Canon pro-grade wide zooms for this trip. You really cant go wrong with any of their high-end glass. People get all hung up on the edge sharpness but honestly, once you stop down to f/8 for those big desert vistas, the difference is basically negligible. Just make sure to get something with that red ring on it because that R5 sensor will show every little flaw in lower-tier stuff. Be careful with the dust out there too, it gets into everything. I would basically just go with the lighter Canon zoom and spend the saved energy on getting to the better viewpoints. Those more compact builds are still super tough and the weather sealing is solid for that desert wind... ngl I think youll appreciate the weight savings way more than the extra stop of light for landscapes.