I expect that putting on a 24T chainring will be easy.

For a few years I used a 24T granny gear, and the stock 42 and 52 chainrings on a road triple for touring. Used that with a 11/32 Sram eight speed cassette. I tried to avoid using the two most cross chained gears on each chainring, thus only used 18 of the gears.

Saddlesores is correct, it is unlikely that your rear derailleur can take up all the slack in the chain. When you are on big and big (52 and 32) and then drop to small and small (24 and 11) that is a difference of 49 teeth. But I found when I did that, I used an 90s vintage XT derailleur (in the photo in my previous post above) and as long as I did not use the two smallest sprockets (11 and 12T) when I used the 24T chainring (which were highly cross chained gears), I had no problem.

I eventually reduced my big ring from 52 to a smaller one, as I almost never used my highest gears, you may consider that too. I mentioned that in my previous post.

Besides a new chain and cassette which I suggested earlier, Saddlesores is right, check your chainrings to see if they are badly worn too. And a different crank may be cheaper if they are.

One thing that is a very messy time consuming bit of maintenance that nobody ever does, but should do, is to remove your rear jockey wheels, clean them, and grease them before reassembly. The two are different have have different parts, so it is best to do one at a time to make sure you do not mix up the parts. Ask your mechanic what she or he thinks about that. I use some removable threadlocker on those two bolts when I reassemble.

I busted a bar end shifter cable last year, that cable had only been in use for four years, but somehow a galvanized cable got into my spares (I try to only buy stainless), I think the hand sweat corroded the cable and caused the failure. Take a very good look at your cables, you might want to replace some of them before the trip. Bar end shifters, stainless is best to avoid corrosion from hand sweat.

This is what your gearing would be like if you switch your 30 to 24, have 42 and 52 chainrings, and use a Sram 11/32 cassette (Shimano cassettes may have different teeth counts). This assumes your two most cross chained gears are not used, they are grayed out.
https://gear-calculator.com/?
GR=DERS&KB=24,42,52&RZ=11,12,14,16,18,21,26,32&UF=2204&TF=72&SL=2.2&UN=MPH&DV=teeth

With your cursor, you can grab a chainring and slide it back and forth to the sides to see how it changes your gears. If you consider a different crank, that way you can see how the different chainring sizes affect your gearing. And you can see if you get a bunch of redundant gears.

If your seatpost has not been out of the bike for a few years, remove it and grease it to prevent dissimilar metal corrosion. That also reduces or prevents rain water from getting inside the bike tubing from that point.

Have a great trip.