Look, I have shot in those old Italian chapels and they are basically caves. I would be very careful about relying on f4 for a wedding ceremony. While the R5 Mark II is a beast with high ISO, you dont want to be pushing 12800 just to get a decent exposure. It can get messy fast. Heres how I see your options based on my time with the R system:

• Canon RF 24-70mm f/2.8L IS USM: This is the safe bet. f2.8 gives you that vital extra stop of light over the f4 which is massive when the sun goes down. The reach is fine, and honestly, you can just crop in a bit with those 45 megapixels if you need more zoom. Its a total workhorse.
• Canon RF 24-105mm f/4L IS USM: The extra reach is tempting for outdoor portraits, but i would suggest avoiding it for the actual ceremony. You might find yourself struggling with grainy shadows. Its a great travel lens, but maybe not for a high-stakes indoor event.
• Canon RF 28-70mm f/2L USM: If you can handle the weight and price, this is the king. It basically replaces a bag of primes. Be warned tho, its heavy and might kill your wrist by the reception. I would suggest getting the 24-70mm f2.8. Just make sure to double check your eye-tracking settings before the big day because that camera has a lot of new tech to learn...

To add to the point above: are you shooting solo? In my experience, just rent the Canon RF 24-70mm f2.8L IS USM. Its way safer for those dark Italian venues.

I've been shooting events for about a decade now and honestly, I was in your exact spot a few years back. I used to think the extra reach of that 105mm was the holy grail for receptions until I actually tried it in a dim ballroom. I ended up staying with the 24-70mm f2.8 and I've been super satisfied with that decision ever since. The depth of field you get at 2.8 just makes the couple pop in a way that f4 never quite manages, especially when things get chaotic during the dancing. The R5 series sensor tech handles noise so well that I dont worry about the ISO as much as I used to, but having that extra stop of light is a lifesaver when the sun goes down or those thick stone walls block every bit of natural light. It just works well for my style... really no complaints once I made the switch. I found that I could always crop in a bit on a high-res sensor if I really needed that extra 30mm, but you cant magically create more light or bokeh in post-production. Quick question tho, are you planning to bring a second body with a fast prime lens for the really dark moments? Also, are these ceremonies gonna be strictly indoors or is there an outdoor garden component where the f4 might actually be fine? Knowing that would change how I would look at the gear situation...