Older tiagra and 105 triples work perfectly fine, I have a 9 spd triple tiagra bike that works great and the fsa cranks bcd allowed for a relatively easy 30t to 26t granny change, with no adjustments needed to fd to maintain shifting.
the step system in the sti works great, that's not the right term, escapes me right now....trim I think.
with the 50/39/26 it's nice to use in hills and I've toured with a 11-32, four panniers.
As noted though, it's finding parts.
Re a 11sp double, bottom line is that you'll appreciate a low gear of 20 or low 20s anyway.
closer rato cassettes are fun to ride though for all the unloaded riding, so I'd keep this in mind also.
check out pathlesspedalled for double gearing hacks options with brifters.
good luck sorting out options