Too much "science" here.

I would opt for the 24T ring. Unless you are small and lacking strength, a 24T front ring with a 32 or 34 or even now a 36 rear large cog will serve you quite well with the existing crankset you have.

The 24-34 granny set-up was what I had when touring Hokkaido (and Britain and Europe and North America) with a load that travelled around the world with us. If you take the advice in the other thread, and travel as light as possible, you will have solved one of the gearing issues -- the lighter the weight, the less need for a really low gear.

Remember also, that as you practise for the trip (and you will be practising, won't you?) and on the ride itself, you will build your strength anyway. That's what happens over 3,600km...