Man, I went through this exact same dilemma before my trip to Arches last year. I was honestly losing sleep over whether to get the f/2.8 or the f/4 for my R5. After hauling gear through the desert for a week, I can tell you I am beyond happy I went with the Canon RF 14-35mm f/4L IS USM. Hiking around Zion is no joke, especially if you're doing stuff like the Narrows or Angels Landing. Every ounce counts when you're climbing. I was worried about the blue hour stuff too, but honestly, with the IBIS on the R5 combined with the lens stabilization, I was pulling off hand-held shots at half a second or longer without any blur. Plus, for those massive canyon views, that extra 1mm on the wide end makes a surprisingly huge difference compared to the Canon RF 15-35mm f/2.8L IS USM. It captures so much more of the sky and foreground. I almost went the adapter route with the Canon EF 16-35mm f/2.8L III USM because I found a deal, but the bulk just isnt worth it on the R system. The RF glass is just sharper across the frame. If you're shooting landscapes, you're usually stopped down to f/8 or f/11 anyway to get everything in focus, so paying extra for f/2.8 just for those blue hour moments doesnt make sense to me anymore. I've had zero complaints with the f/4 version and it fits perfectly in my bag with room to spare.

Been thinking about your dilemma and man, it brings back some annoying memories. I spent a fortune on a high end wide angle a few years back thinking it would magically fix my landscape game, but honestly it was a letdown. I hauled that thing through the desert and the images just werent as sharp as the price tag suggested they should be. It felt like I was paying for a name more than actual performance, which is pretty frustrating when you're counting every penny for travel costs. Nowadays I mostly tell people to just stick with any Canon brand lens in that wide range and not overthink the f-stop too much. The mid-range stuff is usually more than enough for what we do. I found that chasing the most expensive glass usually leads to more neck pain than better photos. Just get a standard Canon zoom and you'll be fine for those canyon views. The lighter ones are so much easier to live with when you're actually out there in the heat and dust anyway.

Saw this earlier but just now responding. Honestly, I had a pretty frustrating time with the heavy f/2.8 zoom I bought for my last canyon trip. I thought I needed that extra stop for the blue hour shots, but it was a massive letdown in the real world.

• The weight was a huge problem. Lugging that massive piece of glass up the trails made me regret the purchase every single mile.
• Unfortunately, the edge distortion was way worse than the reviews said, so I ended up cropping most of my wide shots anyway.
• My old glass with the adapter was even more annoying because it made the whole rig front-heavy and it kept slipping on my tripod. I learned the hard way that chasing the fastest aperture for landscapes is usually a waste of money. The setup I have now is much slower but the images are just as sharp without the back pain. Honestly, the extra stop didnt even matter because I was using a tripod for the blue hour stuff anyway.