The new released RF 16-28mm f/2.8 IS STM lens is a good choice for it compact and lightweight.

And if you are looking for better image quality, take a look at RF 14-35mm f/4L: https://www.bhphotovideo.com/c/search?Ntt=Canon+RF14+35mm+F4+USM+Lens&BI=8941&KBID=10361&SID=12345&DFF=d50

For stunning mountain/coastal landscapes on your R6, the RF 14-35mm f/4L IS USM (around $1300, but sometimes on sale closer to your budget) is hard to beat. It's super sharp and that ultra-wide perspective is perfect for dramatic scenes.

Been thinking about your situation and honestly, after years of hauling gear out to the coast, I've learned that reliability is everything. I used to just chase the sharpest MTF charts, but then I had a barrel lock up on me during a misty morning because the sealing just wasn't up to par for that level of salt spray and moisture ingress. It's basically a nightmare when you're miles from the car and your AF motor starts acting up because of a little grit or humidity. I'm curious though, how often are you planning on shooting in actual adverse conditions? Like, are we talking light drizzle and coastal spray, or strictly blue hour in clear weather? Also, do you already have a 100mm square filter system you're trying to adapt, or are you looking to start fresh? If you're doing long exposures for those coastal shots, the front element design and filter thread compatibility are huge factors for long-term reliability in the field, yeah?