Have you considered that Central America and Mexico have big mountains? . . . Long mountain passes, steep grades .. . roads of unknown quality? It's not like riding from Seattle to Portland!

If you want to gauge your sanity for this ride, go for some rides in the mountains in your area. Load up 30 pounds on the LHT and hit the mountain passes with the 39/53.

For comparison's sake, I know of one rider, Lon Haldeman, the guy who has won the Race Across America I don't know how many times. I believe he's ridden it with a single speed. He's a really strong, smooth rider. He did a trip to Peru on a mtb with a 38t ring and 7 speeds. I don't know the cogs. This is an exceptional athlete doing it. I don't know you so I can't judge, but I consider Haldeman a very rare athlete.

If you do change cranks, and use the 74BCD and one 110BCD ring, I'm not so sure your Ultegra brifters will work or not. Your FD likely will not go low enough to use a small ring as your largest. I'm guessing 40/42 may be the smallest you can go without bottoming out on the frame. You'd need a mtb FD to use two small rings, but then your brifters won't work. See where this is going? These days everything is dependent on each other,mtb stuff doesn't play well with road stuff. . . unless you run friction.

So, either you use one chainring, a compact double(34t minimum), a 74/110 triple(24t min), or your current 39/53(38t min).

In short, I think you're asking for problems that need not exist.