Good morning,
I recently did some accidental and some intentional experiments and found that running 50x11-28 would give me all the gears that I needed.
As you might guess from these ratios I haven't fully bought into the very low gears on offer on many road bikes, 50x28 is the same gear as 42x24 (the normal bottom gear in the 1970s and 1980s) or 34x19.
Unfortunately this was on an 8 speed 11-32 cassette so it was an effective 1x7, big/big wasn't a good combination, meaning that the spacing was not ideal. Had I spent the time and effort I probably could have aligned things so that I could have had 50x32 which would have been useful for a few hundred yards on a couple of routes.
I am unsure if you are on 9/10/11 speed 105 but all of these would fill in the range nicely.
So if you have 10/11 speed 105 and are happy to cross chain big ring to second biggest sprocket you may find that you can just change the cassette and take the front mech off.
Personally I would stop at 32t on the rear as I never have any chain slack issues that might occur with chains long enough to work with a bigger sprocket. Part of this may be because I like to ride down hills so I use 50x11 when available, although if pushed to lose the 11 tooth for a better spread elsewhere I probably would.
I have long moved away from my love of DT shifters for the rear, I doubt that 5 speed STI (never made) would offer much over DT but as the numbers of sprockets went up STI makes these extra sprockets more usable, for me at least.
STI front mech shifters are a different story, I went from straight from DT to Di2 and only recently used mechanical STI for the first time. Both times they were 2000 series on used bikes, one pre and one post the Claris rename and am very underwhelmed.
The frames weren't suitable for a band on shifter and I was reluctant to try and super glue a mounting boss on, I did think about it, but by the time that I found anyone willing to sell me one (
https://cycle-frames.com/collection...8-0850-nova-lever-boss-kit-for-aluminum-frame) I had lost interest.
Bye
Ian