I have dabbled in this endeavor on my tandems, road, and gravel bikes a fair amount. What I have learned is that as long as you can clear the largest cog with the derailleur jockey wheel, you can usual push 2 or 3 links over in capacity. Beyond that, things get a little dicey. Both of our Co-Mo's had 30-39-52 with 11-32 in the rear and I was able to change the 30 to 28 on both bikes (I didn't try 26), however, when I tried to go to a 11-34 in the rear, the capacity was too much and I couldn't get full coverage across all gears. For me, I need to be able to shift into any gear. I may not want to stay there, but I should be able shift into it, then out of it without problem. Many will say "It's OK, I don't need to go into those gear combinations" but my experience is when riding in a hilly area such was we do, that you find you can't anticipate every shift and you don't always know exactly what gear you are in and shifting into something that is going to skip when you're heading uphill is not fun.

With the 28-39-52 set up, I get good performance on my Primera with a Truvative crankset and 5700 RD. On the Carerra, with FSA Gossamer triple crank and Ultegra derailleur, my shifting performance was never good so I just upgraded to SRAM Red Etap with an aftermarket Digirit jockey cage with a 49T capacity. Front is 34-50, rear is 11-42. We lost a little on the high end and gained a little on the low end. This was not cheap, (but I have enough bikes, I could not justify selling the need for another bike to the stoker, but fixing our shifting woes was an easy sell). In full disclosure, I just received and installed the large cage yesterday and we haven't ridden it yet, but it does cover the range the bike has.

In your case, I personally would be hesitant to go to a 26, mainly due to the reasoning I made above. You probably can get away with 28, or you could try dropping on both low and high end such as: 26-39-50/11-32 (or 34) or 28-39-50/11-34 (or 36). Do the math on the capacity numbers and go with your comfort level on shifting issues given your riding conditions. If you and our stoker can handle a drivetrain where the chain starts skipping because it is too loose, then you should be OK (We cannot in our typical riding terrain) Going to a 34 or 36 cog in the rear will likely require a roadlink. I have these on the Carerra, and 2 of my half bikes with good results. The chainrings are realitively cheap and the roadlink will set you back $22 so it is not a lot of money.

If you are considering a mountain derailleur pay close attention to the pull ratios as there is not a lot of compatibility between Shimano road and mountain groups. Shimano has a compatibility chart but I don't believe it covers mixing families (i.e. road and mtn). There is pull ratio data online you should be able to find.

A lot of folks are striving for greater gear ranges and when Shimano came out with their GRX family I was excited that maybe they had recognized the market need, but alas, they did not.