DonValley, thought I'd attach these old gearing charts from my old bike very similar to your bike.
charts are with orig 28t granny, and then with 24t. Both with the 7 speed 13-30 and 28mm tires.
Slightly larger tires (32 or whatever) will bring the gear inch numbers up slightly.
the 21 g.i. that works for me on paved roads with reasonable gradients (maybe up to 15%) worked for me at 25 years old and at 50 years old with about 40lbs of stuff. I'm a skinny guy who doesnt mind climbing and have done a fair amount of it.
I've biked in areas with much steeper hills and would want lower than 19 g.i. that I have on another bike. Travelled in Latin America with about 25lbs or a bit more and 19.5 g.i. was at the limit on some of the really steep stuff, and there were the odd bonkers grades of 20%+ that I reckon that you'd want much lower, especially if you have more like 40lbs.
did these using the Sheldon Brown Gear Calculator I linked to.