Tbh you're right to be skeptical of the older zooms. That 40MP sensor is a beast and it really demands glass with high resolving power. In my experience, the Fujifilm XF 10-24mm f/4 R OIS WR just doesnt hold up at the edges when you're pixel peeping Zion landscapes. It was designed for the older 16-26MP sensors and it shows. If you want something light for those long hikes, grab the Fujifilm XF 14mm f/2.8 R. It’s an older lens but the optical correction is insane—basically zero distortion which is huge for those canyon walls. Another solid move is the Sigma 10-18mm f/2.8 DC DN Contemporary. It’s tiny, weighs next to nothing, and it’s surprisingly sharper than the Fuji zoom on the X-T5. Plus it fits your budget perfectly with room to spare for a good polarizer.

Jumping in because I totally agree about the weight—Zion is no joke. Over the years I've found primes are a safer bet for that 40MP sensors reliability...

• Fujifilm XF 14mm f/2.8 R: Super sharp and tiny for long hikes.
• Fujifilm XF 16mm f/1.4 R WR: Heavier but weather sealing is a lifesaver in dusty canyons. Primes just give more peace of mind for edge-to-edge sharpness.

I went through this exact same dilemma before a trip to Arches last year. Lugging a heavy setup up to Delicate Arch in the heat is basically a nightmare, especially when you've already got a massive water bladder in your bag. I ended up picking up the Sigma 10-18mm f/2.8 DC DN Contemporary and i've been super satisfied with how it performs on my T5. Its honestly tiny... like, fits in the palm of your hand tiny. I had some doubts about it keeping up with the 40mp sensor but the center sharpness is killer. The corners are much better than the old Fuji zoom too. If you're dead set on a prime, check out the Fujifilm XF 14mm f/2.8 R. I've used it for a long time and even on the newer high-res bodies, it works well because the distortion is practically non-existent. It has that mechanical clutch focus too which is great for manual work. Honestly tho, for Zion, you really want that 10mm wideness for stuff like the Narrows where you're looking straight up at those massive walls. The Sigma gives you f2.8 which helps when the light gets low deep in the canyons, and it weighs next to nothing. I think it's like 260g. I did a twelve-mile hike with it and barely noticed it was there. Its easily the best balance of weight and resolution for your budget.