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