Late to the party but I wanted to chime in before your gig. Before throwing money at a new lens, are you sure your current focus issues are purely optical and not a tracking setting or calibration problem on the R6 II? If you need a reliable native performer, here is how a couple of options stack up for that budget:

• Canon RF 85mm f/2 Macro IS STM: This is the standard native choice. Pros are the dual nano USM for fast AF and the macro capability. Cons are the slower f/2 aperture compared to f/1.2 or f/1.4 glass, which limits the bokeh slightly.
• Canon RF 135mm f/1.8 L IS USM: This is likely over your budget unless you find a used one, but the compression is unmatched. It is technically the sharpest portrait lens in the system. The f/2 lens is going to be your best bet for staying under that 1200 limit while keeping perfect AF performance with your body. Honestly, native glass is the only way to avoid the hunting issues you mentioned in high-pressure environments like weddings. Are you shooting mostly wide open or do you stop down a bit for group shots? That might change which focal length makes sense for your Chicago shoot.

Honestly, grab the Canon RF 85mm f/2 Macro IS STM or the Sigma 85mm f/1.4 DG DN Art for Sony mount with an adapter. The sharpness is insane, I'm super satisfied.

Building on the earlier suggestion, you might want to consider how adapters can actually mess with your focus reliability during high-pressure events. I tried using non-native glass for a shoot once and honestly, it missed way too many critical shots. If this wedding is important, I would suggest sticking to native mounts to avoid any firmware nightmares. Make sure to test your gear thoroughly before the big day, because gear failure is the last thing you need.