Is there a reason to replace the components? If they are still good, keep them.
Eight and nine speed Shimano drive trains used the same rear derailleurs, I have been using 1990s vintage long cage XT rear derailleurs for years, have purchased several on the used market. Jockey wheels can be removed, cleaned, re-lubed and re-installed. I do that regularly. If you do that make certain that you do not mix up the upper and lower jockey wheel parts, upper and lower are different. And it is a good idea to use a thread locker when you re-install the jockey wheel bolts. If there is a lot of distance on the jockey wheels they can be replaced with new.
If you buy a new (or used) rear derailleur, if you got a long cage version, then if you had to switch to lower gears on your tour, you could fit a bigger cassette. If you put a new chain on before you start your tour, keep any excess in case you need a few more links if you later put a cassette on that offers lower gearing.
I am clueless on external bearing bottom brackets, I assume that is what you have and I have no comments on those, I use something different.
You were silent on wheels, if your rear wheel is cup and cone, open it up and if the ball bearings looks clean and shiny, add grease before closing it up. Cartridge bearing hub, I have no comments.
Older Shimano brifters can suddenly stop working. If yours is only six years old, it probably is good. But if it is older, if the internals can be cleaned and lubed, you may want to do so. A friend of mine had his rear 9 speed Shimano brifter die on him a month before he started a 3000 mile tour, he quickly switched to bar end shifters. I think his brifter was over a decade old when it died. The tour he was getting ready for was a group tour, on the tour one of the group had her rear brifter die, she finished the tour with three speeds because she had a triple crank but lost the ability to shift the rear. I suspect two out of a dozen people with rear Shimano rear brifter problems are more problems than average, but it does highlight that something you expect to last forever sometimes does not. They can't be disassembled, but you might be able to clean and lube it.
I made no comments on expendables like chain, cassette, chainrings, cables, I assume you have that stuff under control.
