I spent a few years shooting with heavy setups and honestly, my back still regrets it. The R8 is so compact that sticking a massive lens on the front feels like youre trying to balance a sledgehammer on a pencil. I moved over to the f/4 line for my wide shots and havent looked back since the image quality is basically identical for landscapes anyway. Heres a couple things to think about:

• Canon RF 14-35mm f/4L IS USM is the most logical choice here. It fits your budget perfectly at around $1400. Being an L lens, it has the weather sealing you want for outdoor shots, and that extra 1mm at the wide end is a lifesaver for architecture.
• Canon RF 15-30mm f/4.5-6.3 IS STM is the budget pick. Its very light, but you lose that professional build quality and the constant aperture. Id personally go for the 14-35mm. Its a safer bet for long-term use and wont make your R8 feel totally unbalanced like the f/2.8 version does.

honestly the R8 is such a lightweight beast that you really gotta think about the balance of the whole rig before you drop that kind of cash - it can get front-heavy real fast with the chunky professional glass. since youre looking at architecture too do you actually need that super wide 14 or 15mm end for tight interiors or are you finding that 20mm or 24mm is usually enough for most of your exterior shots? basically I'm wondering if you prioritize that low-light f/2.8 speed for handheld work or if youre usually on a tripod where a slower and way cheaper lens would perform just as well because tbh for landscapes youre usually stopped down to f/8 anyway so you might be overpaying for features you wont even use!! let us know what your typical setup looks like so we can narrow it down

Get the new released RF 16-28mm f/2.8 IS STM lens. https://www.bhphotovideo.com/c/search?Ntt=Canon+RF16+28mm+F2+8+is+STM&BI=8941&KBID=10361&SID=12345&DFF=d50