look, this should be really easy, barely an inconvenience.

you've got the bike, you've got racks, you've got packs.
best of all, the bike already has some gears.

get you a notebook, and note down the chain rings and the cogs.

load up your bike as you would for your tour or commute.
ride around and see just which gears are the bestest for
various types of terrain and wind conditions.

mark in red the bestestest gear combinations.

and now hit the interwebs, go to some of the recommended gear
calculators. enter in your current setup, including tire sizes,
and see what your current setup gives you in terms of gear inches
in all combinations, and which your favorite gear inches are.

now play with the calculator, figger out which chainring and
cog combinations will get you those same gear inches, and
can you give you more top and/or low end.

then you can see what you'd need to do change to get
a little higher or lower........