I've entered Deano's Most Excellent off-road 100, but I'm a right fat fucker at the moment, so I need to gear my CAADX down from the 11-28 11sp cassette it comes with. I'm looking at a Shimano 11-42 MTB cassette, but I'm kind of concerned there won't be the capacity for that. The front has 46/36.
Note that the rear mech of 105, Ultegra, and Dura Ace are interchangable (some rumour the Tiagra 4700 to also be interchangable). I think there is an Ultegra rear mech that will accept a 34 at the rear. I have the GS version of the tiagra 4700, and I have a 11-34 rear cassette.
Of all the parts that can be changed, the rear mech is relatively* affordable.
*for values of affordable
* About the gearing, that is - I know I'm right to be concerned about being a right fat fucker.
As a fellow person of substantial mass, I can recommend ambitious bike adventures as a way to lose weight. I lost 3.5kg in a single 7 day trip through Germany...
The alternative would be to downsize the chainrings, presumably a non-Shimano option would be needed.
I ran a 1x setup at the Yr Elenydd recently, and found the 42x11 to be more than adequate for the faster bits of riding (I freewheel in any case when going suitably fast!), and the 42x42 was good for the uphill bits. I'll be trying out a 36 tooth chainring if I make it up for Deano's ride.
So a chainring combination of 40/26 or similar might work ok for you, as an alternative option.
So on my frankenbike, which is based on a Genesis Vagabond, I couldn't fit my planned 50/34 front chainset on, so ended up with a mountain bike chainset. I have a Deore 40/28 on the front, with the matching front derailure, which is driven by tiagra 4700 shifters. Officially this is a combo that Will Not Work™, but the reality is, I've done 2760km on this setup and it's worked so far.
Even with a 28-34 bottom gear, I still struggle on some hills, but that's due to the aforementioned excess kummerspek, and I hope to resolve this in due course...
J