Actually, I disagree with the f2.8 zoom logic! While the Canon EF 16-35mm f/2.8L III USM is fast, the Canon RF 14-35mm f/4L IS USM is an absolute beast for landscape sharpness. For your astro fix, just pair it with the Canon RF 16mm f/2.8 STM. It's super light and youll still stay under that $1500 budget! TL;DR: Native RF glass wins for corner sharpness and weight.

I tried the whole adapter thing for my first big trip and honestly it was not as good as expected. Using the Canon Mount Adapter EF-EOS R with older glass like the Canon EF 16-35mm f/2.8L III USM just felt off. The setup is way too front-heavy and I constantly worried about the weather sealing failing at the connection point. Its just one more thing that can go wrong when youre out in the dust at Zion. Unfortunately, even the native Canon RF 14-35mm f/4L IS USM had issues that annoyed me. The distortion at 14mm is pretty wild before the camera software fixes it, which really affects the corner sharpness for those big canyon shots. I found it pretty disappointing for an L-series lens at that price point. If you want my honest advice as someone who prioritizes reliability:

• Stick to native RF glass only. The extra communication pins between the lens and the body make the autofocus much more reliable than using an adapter.
• Forget the f/2.8 for landscapes. You are gonna be stopped down to f/8 or f/11 most of the time anyway, so the f/4 is plenty and saves your back on those long hikes. Basically, the EF route is just too much of a gamble for a big trip. Id rather have a lighter bag and a secure mount than that extra stop of light for astro... i just dont trust the adapter for a week of heavy use.

Ive been very satisfied with native glass for my workflow recently. It works well and provides better balance. Are you prioritizing edge sharpness or aperture range for those Zion landscapes?

My buddy told me the exact same thing last week. Guess he was right lol.

Regarding what #4 said about "My buddy told me the exact same thing...", I totally agree that native glass is the way to go for the R system, but you might want to consider some technical caveats before dropping the cash. While the Canon RF 14-35mm f/4L IS USM is incredibly sharp, be careful with the 14mm end. It has massive native barrel distortion that requires heavy software correction. If you use a specialized RAW editor that doesnt have the latest profiles, you might see some dark corners. I would suggest also looking at a dedicated prime if the astro part is really nagging at you. The Laowa 15mm f/2 Zero-D for RF mount is fully manual but its a landscape beast with almost zero distortion, and it fits comfortably in that $1500 budget alongside a used RF wide zoom. Just make sure to double check your tripod stability because Zion can get pretty windy in the canyons. Tbh the weight savings of avoiding the Canon Mount Adapter EF-EOS R will make those long hikes way more bearable.

Just saw this thread and honestly, I feel your pain so much. Trying to find the perfect wide-angle for the R system without spending a fortune or carrying a brick of an adapter is just the worst. I spent weeks stressing over this exact dilemma, trying to DIY some filter holders and rigging up different setups, and it is just a massive headache. Either you get giant adapted glass or you have to deal with heavy software warping. You definitely might want to consider how much gear you actually want to lug around Zion. To help figure this out, how much do you actually plan to shoot astro compared to daytime landscapes next week? Like, is astro just a one-night thing or a main goal? Also, make sure to think about how you will display them... are you planning to do massive prints, or is this mostly for web? Let me know, always down to help you bounce more ideas around before you head out!