tbh the Canon RF 24-105mm f/4L IS USM is gonna be your best bet here. i've shot plenty of events with it and while f/4 sounds slow, the Canon EOS R6 is seriously good at high ISO so dont sweat it too much. you'll appreciate that extra reach at 105mm way more than you think during the ceremony. one thing tho... be careful if the Montana venue is one of those dark timber barns. you might want to consider picking up a cheap Canon RF 50mm f/1.8 STM too just in case the light totally tanks during the reception. it fits in your pocket and costs almost nothing. make sure to keep your shutter speed up during the dancing even if you have to crank the ISO... grain is better than a blurry mess! that 24-105 is a workhorse, just pull the trigger and get used to the weight before next thursday.

Honestly, for a wedding in Montana with that budget, you really should just grab the f/4 zoom. I have been using mine for a while now and I am perfectly satisfied with how it performs across the board. The stabilization is solid and honestly, the R6 sensor is a total beast in low light, so that extra stop of light from a 2.8 really isnt the dealbreaker people make it out to be. Here is what you need to watch out for though:

• Dont try to shoot at f/4 if you dont have a flash, especially in a dark barn setting.
• Avoid cranking your shutter speed unnecessarily high if the lighting is dim; it will just introduce noise.
• Make sure you practice your low light focusing technique before the event. It works well for almost everything I throw at it, and honestly, the extra reach is way more useful for candid shots than a slightly wider aperture would be. Just get the gear and get to shooting, you will be fine.